Add support for `@layer`

[romainmenke]

This has an existing pull request : #7

• specification : https://drafts.csswg.org/css-cascade-5/#layering
• wpt : https://wpt.fyi/results/css/css-cascade?label=master&label=experimental&aligned&q=layer
• mdn docs : https://developer.mozilla.org/en-US/docs/Web/CSS/@layer
• shipped in all browsers

This feature is very tricky to implement safely without a working test suite

[vs-code-engineering]

Sharing media and context from merged PR #52 for verification:

Part of #20
Related to #7

This adds TextMate grammar support for CSS cascade layers.

Covered syntax:

• @layer reset, framework.theme;
• @layer framework { ... }
• anonymous layers: @layer { ... }
• nested layers: @layer foo { @layer bar { ... } }
• @import "theme.css" layer;
• @import url("theme.css") layer(framework.theme);

This intentionally keeps the scope limited to cascade layer syntax. Broader latest @import syntax support from #20 can be handled separately.

Testing

npm test

Result:

# tests 208
# pass 202
# fail 0
# skipped 6

Screenshots

Before:

After:

Source PR: #52