SCSS: Support parameterized pseudo-classes in selectors

[dweep-js]

Fixes #3740

Problem

SCSS selectors containing a parameterized pseudo-class were not
highlighted as selectors:

foo :foo() bar {

}

:foo() broke the match entirely — only bar was left as an
unmatched token, while foo :foo() fell out of the selector token.
The equivalent selector without parens (foo :foo bar {}) worked fine.

Root cause

Prism.languages.scss.selector.pattern explicitly excludes ( and )
from the character class that makes up a selector:

[^@;{}()\s]

This exclusion exists on purpose: the same pattern is reused (via
atrule's $rest: 'scss') to tokenize the inside of at-rules like
@mixin and @include, and parens there belong to the argument list,
not to a selector. A naive fix — just deleting () from the exclusion
class, as originally suggested in the issue — makes the selector
pattern swallow at-rule argument lists too, which breaks highlighting
of code like:

@mixin prefix($property, $value, $prefixes) {
}

(prefix should tokenize as a function; with the naive fix it gets
absorbed into a spurious selector token instead, as flagged by
@Golmote in the issue thread.)

Fix

Instead of opening up () unconditionally, this adds one new,
narrowly-scoped alternative to the selector pattern: a balanced
parenthesized group, but only when it directly follows a
pseudo-class name (:name(...) or ::name(...)):

:{1,2}[-\w]+\((?:[^()]|\([^()]*\))*\)

This matches :foo(), :nth-child(2n+1), :not(.active), etc., while
leaving the existing exclusion of bare (/) intact everywhere else —
so @mixin/@include argument lists (which are never preceded by a
colon) are unaffected.

Testing

Added a new case to tests/languages/scss/selector_feature.test:

foo :foo() bar {}

Verified the fix two ways:

1. Regression-proof: temporarily reverted just the scss.js
   change (git stash push -- src/languages/scss.js) while keeping
   the new test case, and confirmed selector_feature fails against
   the old pattern — foo :foo() bar {} tokenizes as
   property/punctuation/function/punctuation/punctuation/
   selector("bar ") instead of one clean selector token. This
   reproduces the exact bug from the issue.
2. Restored the fix and confirmed all tests pass again
   (npm run test:languages -- --language=scss, 15 passing).
3. Manually re-checked Golmote's @mixin prefix($property, $value, $prefixes) {} counter-example against the new pattern to confirm
   prefix still tokenizes correctly as a function and is not
   swallowed into a selector — the specific regression that sank
   the original one-line workaround in the issue thread.

Scope

One-line regex change in src/languages/scss.js, plus one new test
case appended to the existing selector_feature.test (no existing
cases modified).

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