Add @layer at-rule support to Lucius parser

[jeongoon]

Base: yesodweb/shakespeare master
Head: jeongoon/shakespeare support-at-layer

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Summary

CSS @layer allows explicit cascade layer ordering for better CSS organization. This PR adds support for all three valid CSS forms:

• Block form: @layer utilities { .class { prop: val; } }
• Declaration form: @layer utilities; or @layer a, b, c;
• Anonymous block: @layer { .class { prop: val; } }

Additionally, this PR enables recursive nesting of at-rules inside each other. Previously, at-rule blocks (@media, @supports, etc.) could only contain regular CSS blocks. Modern CSS commonly requires nesting at-rules, e.g.:

@layer components {
    @media (max-width: 600px) {
        .btn { font-size: 1.2rem; }
    }
}

Changes

• Added layer parser to Text/Internal/Lucius.hs
• Changed TopAtBlock to hold [TopLevel a] instead of [Block a] in Text/Internal/Css.hs
• Updated parseBlocks to parse nested at-rules (media, supports, layer, topAtBlock) in addition to regular blocks
• Updated compressTopLevel, cssUsedIdentifiers, renderCss, cssRuntime, topLevelsToCassius, and luciusRTInternal for recursive TopLevel handling
• renderCss now supports nested indentation via goWith indent pattern
• Added 16 test cases (8 unordered + 8 ordered) to test/Text/CssSpec.hs

Implementation notes

• Uses try for backtracking between block and declaration forms for @layer
• Uses many instead of many1 for the selector to support anonymous layers (empty selector)
• Follows the same pattern as existing @media and @supports parsers
• Nested at-rules reuse the same parseBlocks → at-rule parser path recursively

Backwards compatibility

TopAtBlock is exported from Text.Internal.Css which is marked "not stable API". Code that pattern-matches on TopAtBlock will need to update the third field from [Block a] to [TopLevel a].

Test plan

• All 285 existing tests pass (293 total with new tests)
• New @layer tests cover block, declaration, multiple declaration, and anonymous forms
• New nested at-rule tests: @media inside @layer, @layer inside @media, @supports inside @layer, @media inside @media
• Tests added for both ordered and unordered Lucius variants

References

• MDN: @layer
• CSS Cascade Layers spec

[jeongoon]

some tests fails only because it fails to download the compilers.