Recover non-lemnos themes (#902)

Changes:

* Custom Nushell `color_config`'s (non-Lemnos) are moved to the
`./src/custom-nu-themes` directory and used to generate the main theme
files when doing a `./make.nu`
* `make.nu` and other scripts (e.g. preview) have been moved to the
`src` directory
* A separate `README` for the `src` scripts
* Main theme `README` updated for clarity
* Reordered the theme template, so all themes were regenerated. I'd like
to stop doing this ;-)

@amtoine Thanks for your suggestions on the `README`. I've pretty
extensively changed the flow, so please re-review and let me know how it
looks. Also note that I haven't personally tested with Nupm, so please
especially review this section. Thanks!

@fdncred CC

Still planning on working on the preview scripts next. They'll need only
slight changes to run (probably a path change), but they can be
simplified a bit now, I hope. At the same time, I'll complicate them in
other ways ;-).

Note that there is a bug in both `nushell-dark` and `nushell-light` that
prevented the terminal colors from being updated regardless. You can see
that in the existing screenshots, where both those themes simply re-use
the previous theme (alphabetically's) background. I haven't fixed this
yet.

themes/nu-themes/tokyo-moon.nu

@@ -0,0 +1,119 @@
+# Retrieve the theme settings
+export def main [] { return {
+    separator: "#a9b1d6"
+    leading_trailing_space_bg: { attr: "n" }
+    header: { fg: "#9ece6a" attr: "b" }
+    empty: "#7aa2f7"
+    bool: {|| if $in { "#7dcfff" } else { "light_gray" } }
+    int: "#a9b1d6"
+    filesize: {|e|
+        if $e == 0b {
+            "#a9b1d6"
+        } else if $e < 1mb {
+            "#7dcfff"
+        } else {{ fg: "#7aa2f7" }}
+    }
+    duration: "#a9b1d6"
+    date: {|| (date now) - $in |
+        if $in < 1hr {
+            { fg: "#f7768e" attr: "b" }
+        } else if $in < 6hr {
+            "#f7768e"
+        } else if $in < 1day {
+            "#e0af68"
+        } else if $in < 3day {
+            "#9ece6a"
+        } else if $in < 1wk {
+            { fg: "#9ece6a" attr: "b" }
+        } else if $in < 6wk {
+            "#7dcfff"
+        } else if $in < 52wk {
+            "#7aa2f7"
+        } else { "dark_gray" }
+    }
+    range: "#a9b1d6"
+    float: "#a9b1d6"
+    string: "#a9b1d6"
+    nothing: "#a9b1d6"
+    binary: "#a9b1d6"
+    cellpath: "#a9b1d6"
+    row_index: { fg: "#9ece6a" attr: "b" }
+    record: "#a9b1d6"
+    list: "#a9b1d6"
+    block: "#a9b1d6"
+    hints: "dark_gray"
+    search_result: { fg: "#f7768e" bg: "#a9b1d6" }
+
+    shape_and: { fg: "#bb9af7" attr: "b" }
+    shape_binary: { fg: "#bb9af7" attr: "b" }
+    shape_block: { fg: "#7aa2f7" attr: "b" }
+    shape_bool: "#7dcfff"
+    shape_custom: "#9ece6a"
+    shape_datetime: { fg: "#7dcfff" attr: "b" }
+    shape_directory: "#7dcfff"
+    shape_external: "#7dcfff"
+    shape_externalarg: { fg: "#9ece6a" attr: "b" }
+    shape_filepath: "#7dcfff"
+    shape_flag: { fg: "#7aa2f7" attr: "b" }
+    shape_float: { fg: "#bb9af7" attr: "b" }
+    shape_garbage: { fg: "#FFFFFF" bg: "#FF0000" attr: "b" }
+    shape_globpattern: { fg: "#7dcfff" attr: "b" }
+    shape_int: { fg: "#bb9af7" attr: "b" }
+    shape_internalcall: { fg: "#7dcfff" attr: "b" }
+    shape_list: { fg: "#7dcfff" attr: "b" }
+    shape_literal: "#7aa2f7"
+    shape_match_pattern: "#9ece6a"
+    shape_matching_brackets: { attr: "u" }
+    shape_nothing: "#7dcfff"
+    shape_operator: "#e0af68"
+    shape_or: { fg: "#bb9af7" attr: "b" }
+    shape_pipe: { fg: "#bb9af7" attr: "b" }
+    shape_range: { fg: "#e0af68" attr: "b" }
+    shape_record: { fg: "#7dcfff" attr: "b" }
+    shape_redirection: { fg: "#bb9af7" attr: "b" }
+    shape_signature: { fg: "#9ece6a" attr: "b" }
+    shape_string: "#9ece6a"
+    shape_string_interpolation: { fg: "#7dcfff" attr: "b" }
+    shape_table: { fg: "#7aa2f7" attr: "b" }
+    shape_variable: "#bb9af7"
+
+    background: "#1a1b26"
+    foreground: "#c0caf5"
+    cursor: "#c0caf5"
+}}
+
+# Update the Nushell configuration
+export def --env "set color_config" [] {
+    $env.config.color_config = (main)
+}
+
+# Update terminal colors
+export def "update terminal" [] {
+    let theme = (main)
+
+    # Set terminal colors
+    let osc_screen_foreground_color = '10;'
+    let osc_screen_background_color = '11;'
+    let osc_cursor_color = '12;'
+        
+    $"
+    (ansi -o $osc_screen_foreground_color)($theme.foreground)(char bel)
+    (ansi -o $osc_screen_background_color)($theme.background)(char bel)
+    (ansi -o $osc_cursor_color)($theme.cursor)(char bel)
+    "
+    # Line breaks above are just for source readability
+    # but create extra whitespace when activating. Collapse
+    # to one line
+    | str replace --all "\n" ''
+    | print $in
+}
+
+export module activate {
+    export-env {
+        set color_config
+        update terminal
+    }
+}
+
+# Activate the theme when sourced
+use activate