[scss] Support bracketed lists

[Rusty Raven (kakiuwang-ui)]

Fixes #231
Fixes #399

Problem

Sass bracketed lists are reported as syntax errors:

.a { $x: [1px, 2px]; }
//        ^ ] expected
//               ^ semi-colon expected
//                 ^ at-rule or selector expected

The only production that accepts [ in a value position is _parseNamedLine, which implements the CSS named-line syntax and therefore only accepts a bare sequence of identifiers. That happens to cover [a b c], so space-separated lists of idents already worked — but anything else (commas, numbers, strings, variables, interpolation, function calls, nesting) fell through and produced a cascade of errors.

Fix

Override _parseNamedLine in SCSSParser so the brackets hold a full expression. A Sass bracketed list is a superset of the CSS named-line syntax, so the one production covers both; CSS and LESS are untouched.

Now parsing cleanly

$x: [];                            // empty
$x: [a];
$x: [a, b, c];                     // comma separated
$x: [a, b,];                       // trailing comma
$x: [1px, 2em, 3%];
$x: [$a, $b];
$x: [#{$a}, b];                    // interpolation
$x: [[a, b], [c d]];               // nested
$x: [math.div(6, 2), 2];           // function call
$m: (k: [a, b], j: [c]);           // as a map value
@mixin m($x: [a, b]) { }           // as a default parameter
@each $i in [a, b] { }
.a { color: list.nth([a, b], 2); } // as an argument

Still reported as errors:

$x: [a, b;   // ] expected
$x: a];      // semi-colon expected

And the CSS named-line syntax keeps working in both CSS and SCSS:

.a { grid-template-columns: [full-start] 1fr [full-end]; }
.a { grid-template-rows: [r1-start r2] 25% [r2-end]; }

Testing

npm run test — 712 passing, 0 failing (710 before, plus the new cases).

New tests in src/test/scss/parser.test.ts: a Bracketed list block covering the forms above plus the RightSquareBracketExpected error case, a Bracketed list in a stylesheet block covering the end-to-end usages from both issues, and two cases added to VariableDeclaration.