make the themes a module (#648)

related to
- https://github.com/nushell/nupm/pull/33

## description
with https://github.com/nushell/nupm/pull/33, Nupm can now install
separate modules from a single package 🥳

the `nu_scripts` are a package and looks like they contain a few
potential standalone modules:
- the themes
- the hooks
- the custom completions
- ...

this PR is a proposal to move `themes/` to `nu-themes/` and add
`./nu-themes/` as a module to the `nu-scripts` package.
that way when running `nupm install --path .` in the root of the
`nu_scripts`, a `nu-themes` module will be installed in
`$env.NUPM_HOME/modules`.

then one can run
```nushell
use nu-themes/themes/nushell-dark.nu
```
and next get the theme with
```nushell
nushell-dark
```

themes/nu-themes/mexico-light.nu

@@ -0,0 +1,82 @@
+export def main [] { return {
+    separator: "#383838"
+    leading_trailing_space_bg: { attr: "n" }
+    header: { fg: "#538947" attr: "b" }
+    empty: "#7cafc2"
+    bool: {|| if $in { "#4b8093" } else { "light_gray" } }
+    int: "#383838"
+    filesize: {|e|
+        if $e == 0b {
+            "#383838"
+        } else if $e < 1mb {
+            "#4b8093"
+        } else {{ fg: "#7cafc2" }}
+    }
+    duration: "#383838"
+    date: {|| (date now) - $in |
+        if $in < 1hr {
+            { fg: "#ab4642" attr: "b" }
+        } else if $in < 6hr {
+            "#ab4642"
+        } else if $in < 1day {
+            "#f79a0e"
+        } else if $in < 3day {
+            "#538947"
+        } else if $in < 1wk {
+            { fg: "#538947" attr: "b" }
+        } else if $in < 6wk {
+            "#4b8093"
+        } else if $in < 52wk {
+            "#7cafc2"
+        } else { "dark_gray" }
+    }
+    range: "#383838"
+    float: "#383838"
+    string: "#383838"
+    nothing: "#383838"
+    binary: "#383838"
+    cellpath: "#383838"
+    row_index: { fg: "#538947" attr: "b" }
+    record: "#383838"
+    list: "#383838"
+    block: "#383838"
+    hints: "dark_gray"
+    search_result: { fg: "#ab4642" bg: "#383838" }
+
+    shape_and: { fg: "#96609e" attr: "b" }
+    shape_binary: { fg: "#96609e" attr: "b" }
+    shape_block: { fg: "#7cafc2" attr: "b" }
+    shape_bool: "#4b8093"
+    shape_custom: "#538947"
+    shape_datetime: { fg: "#4b8093" attr: "b" }
+    shape_directory: "#4b8093"
+    shape_external: "#4b8093"
+    shape_externalarg: { fg: "#538947" attr: "b" }
+    shape_filepath: "#4b8093"
+    shape_flag: { fg: "#7cafc2" attr: "b" }
+    shape_float: { fg: "#96609e" attr: "b" }
+    shape_garbage: { fg: "#FFFFFF" bg: "#FF0000" attr: "b" }
+    shape_globpattern: { fg: "#4b8093" attr: "b" }
+    shape_int: { fg: "#96609e" attr: "b" }
+    shape_internalcall: { fg: "#4b8093" attr: "b" }
+    shape_list: { fg: "#4b8093" attr: "b" }
+    shape_literal: "#7cafc2"
+    shape_match_pattern: "#538947"
+    shape_matching_brackets: { attr: "u" }
+    shape_nothing: "#4b8093"
+    shape_operator: "#f79a0e"
+    shape_or: { fg: "#96609e" attr: "b" }
+    shape_pipe: { fg: "#96609e" attr: "b" }
+    shape_range: { fg: "#f79a0e" attr: "b" }
+    shape_record: { fg: "#4b8093" attr: "b" }
+    shape_redirection: { fg: "#96609e" attr: "b" }
+    shape_signature: { fg: "#538947" attr: "b" }
+    shape_string: "#538947"
+    shape_string_interpolation: { fg: "#4b8093" attr: "b" }
+    shape_table: { fg: "#7cafc2" attr: "b" }
+    shape_variable: "#96609e"
+
+    background: "#f8f8f8"
+    foreground: "#383838"
+    cursor: "#383838"
+}}