Simplify theme usage (#896)

* Primary change: Simplifies theme usage by adding an `export-env` to
each theme so that `$env.config.color_config` is automatically set when
importing the module.

  Rather than the old:

  ```nu
  use nu-themes/nushell-dark.nu
  $env.config.color_config = (nushell-dark)
  ```

  It's now a single-step:

  ```nu
  use nu-themes/nushell-dark.nu
  ```

* Updates `make.nu` to create the theme files (uses the previously added
`str dedent` for clean output).
* A couple (literally, 2) of the remote themes did not have all
necessary colors and failed. Updated `make.nu` to continue on error, but
report the failures.
* Updated `README.md`

themes/nu-themes/tokyo-moon.nu

@@ -1,82 +0,0 @@
-export def main [] { return {
-    separator: "#828bb8"
-    leading_trailing_space_bg: { attr: "n" }
-    header: { fg: "#c3e88d" attr: "b" }
-    empty: "#82aaff"
-    bool: {|| if $in { "#86e1fc" } else { "light_gray" } }
-    int: "#828bb8"
-    filesize: {|e|
-        if $e == 0b {
-            "#828bb8"
-        } else if $e < 1mb {
-            "#86e1fc"
-        } else {{ fg: "#82aaff" }}
-    }
-    duration: "#828bb8"
-    date: {|| (date now) - $in |
-        if $in < 1hr {
-            { fg: "#ff757f" attr: "b" }
-        } else if $in < 6hr {
-            "#ff757f"
-        } else if $in < 1day {
-            "#ffc777"
-        } else if $in < 3day {
-            "#c3e88d"
-        } else if $in < 1wk {
-            { fg: "#c3e88d" attr: "b" }
-        } else if $in < 6wk {
-            "#86e1fc"
-        } else if $in < 52wk {
-            "#82aaff"
-        } else { "dark_gray" }
-    }
-    range: "#828bb8"
-    float: "#828bb8"
-    string: "#828bb8"
-    nothing: "#828bb8"
-    binary: "#828bb8"
-    cellpath: "#828bb8"
-    row_index: { fg: "#c3e88d" attr: "b" }
-    record: "#828bb8"
-    list: "#828bb8"
-    block: "#828bb8"
-    hints: "dark_gray"
-    search_result: { fg: "#ff757f" bg: "#828bb8" }
-
-    shape_and: { fg: "#c099ff" attr: "b" }
-    shape_binary: { fg: "#c099ff" attr: "b" }
-    shape_block: { fg: "#82aaff" attr: "b" }
-    shape_bool: "#86e1fc"
-    shape_custom: "#c3e88d"
-    shape_datetime: { fg: "#86e1fc" attr: "b" }
-    shape_directory: "#86e1fc"
-    shape_external: "#86e1fc"
-    shape_externalarg: { fg: "#c3e88d" attr: "b" }
-    shape_filepath: "#86e1fc"
-    shape_flag: { fg: "#82aaff" attr: "b" }
-    shape_float: { fg: "#c099ff" attr: "b" }
-    shape_garbage: { fg: "#FFFFFF" bg: "#FF0000" attr: "b" }
-    shape_globpattern: { fg: "#86e1fc" attr: "b" }
-    shape_int: { fg: "#c099ff" attr: "b" }
-    shape_internalcall: { fg: "#86e1fc" attr: "b" }
-    shape_list: { fg: "#86e1fc" attr: "b" }
-    shape_literal: "#82aaff"
-    shape_match_pattern: "#c3e88d"
-    shape_matching_brackets: { attr: "u" }
-    shape_nothing: "#86e1fc"
-    shape_operator: "#ffc777"
-    shape_or: { fg: "#c099ff" attr: "b" }
-    shape_pipe: { fg: "#c099ff" attr: "b" }
-    shape_range: { fg: "#ffc777" attr: "b" }
-    shape_record: { fg: "#86e1fc" attr: "b" }
-    shape_redirection: { fg: "#c099ff" attr: "b" }
-    shape_signature: { fg: "#c3e88d" attr: "b" }
-    shape_string: "#c3e88d"
-    shape_string_interpolation: { fg: "#86e1fc" attr: "b" }
-    shape_table: { fg: "#82aaff" attr: "b" }
-    shape_variable: "#c099ff"
-
-    background: "#222436"
-    foreground: "#c8d3f5"
-    cursor: "#c8d3f5"
-}}