Fix CSS highlighting desync when a nested rule precedes a custom property

[srikanthananthula63053]

Summary

• Fixes CSS highlighting breaks when a nested rule comes before a custom property in the same block vscode-css#54 — CSS syntax highlighting desyncs when a nested rule inside a block is followed by a custom property, e.g. #a { .b { gap: 1px; } --c: 1px; }.
• Root cause: rule-list-innards had no pattern for a nested rule block, so the enclosing block's own end pattern (}) matched the first } encountered — even one belonging to a nested rule — closing the outer meta.property-list.css scope prematurely. Everything after the nested rule then fell back to raw, uncolored meta.selector.css text.
• rule-list-innards now recognizes nested selectors via #selector, and rule-list re-enters the grammar ($self) so a nested { ... } opens its own properly balanced scope. selector's begin pattern is tightened (with negative lookaheads) so it only matches when a selector is actually followed by {, not an ordinary property: value; declaration, since it now also runs inside a property list where it competes with declaration parsing.

Test plan

• Tokenized the exact repro from the issue with vscode-textmate + vscode-oniguruma directly against the grammar file — confirmed the nested rule and the trailing custom property both highlight correctly, with no scope leaking past the nested block's own closing brace.
• Tokenized the previously-"working" reverse order (--c before .b { ... }) — still correct, and no longer relies on incidental brace-position luck.
• Diffed full-file tokenization (old grammar vs. patched grammar) across large, untouched real CSS files in the repo — zero differences.
• Diffed tokenization of src/vs/workbench/contrib/chat/browser/widget/media/chat.css (4600+ lines) — the only differences were at genuine nested-rule sites, including an already-shipping nested rule (.codicon { font-size: inherit; }) that this same bug was silently mis-highlighting; it now renders correctly, and the file continues to tokenize cleanly through EOF afterward.
• Additional manual cases verified: pseudo-class nesting (&:hover), tag-name nesting, attribute-selector nesting, deeply nested rules, nested @media, and var() values — all unaffected/correct.
• Added an automated regression test: extensions/vscode-colorize-tests/test/colorize-fixtures/test-css-nesting.css, covering the exact issue repro plus custom-property-before-nested-rule, pseudo-class nesting with a trailing custom property, and a nested rule inside @media. Ran via npm run test-extension -- -l vscode-colorize-tests --grep test-css-nesting — passing, with the checked-in colorize-results/colorize-tree-sitter-results snapshots locking in the corrected tokenization (e.g. --c after the nested rule now scopes as variable.css instead of falling back to unstyled selector text). Any future regression in this grammar will now fail this test.

Files Changed

• extensions/css/syntaxes/css.tmLanguage.json — grammar fix (see Root Cause above).
• extensions/vscode-colorize-tests/test/colorize-fixtures/test-css-nesting.css — new regression fixture.
• extensions/vscode-colorize-tests/test/colorize-results/test-css-nesting_css.json and .../colorize-tree-sitter-results/test-css-nesting_css.json — checked-in expected tokenization snapshots for the new fixture.

[alexr00]

Thank you for the PR, but we pull our css grammar from https://github.com/microsoft/vscode-css, so all PRs should go there.