fix(themes): resolve open-color map-get deprecation warning

[ppvg]

The use of map-get in open-color.scss without @useing sass:map results in deprecation warnings when building a project that uses Manon.

Example deprecation warning

Deprecation Warning [global-builtin]: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use map.get instead.

More info and automated migrator: https://sass-lang.com/d/import

   ╷
36 │ $oc-gray-4: map-get($oc-gray-list, "4");
   │             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    node_modules/@minvws/manon-themes/icore-open/color-scheme/open-color.scss 36:13  @forward
    node_modules/@minvws/manon-themes/icore-open/color-scheme/_index.scss 6:1        @use
    node_modules/@minvws/manon-themes/icore-open/application-base.scss 4:1           @use
    node_modules/@minvws/manon-themes/icore-open/theme.scss 10:1                     @forward
    node_modules/@minvws/manon-themes/icore-open/_index.scss 1:1                     @use
    resources/scss/manon/_theme.scss 1:1                                             @use
    resources/scss/manon/_index.scss 2:1                                             @use
    resources/scss/app.scss 2:1                                                      root stylesheet

Open-color was last updated in 2021. I don't expect it to be updated to fix this. We've already inlined open-color.scss in this repo, so it makes sense to fix it here. The relevant fix is in c14129a. The rest is some SPDX housekeeping and code formatting.

[randh1r]

lgtm