fix: remove root & inside functional pseudo-classes

[spokodev]

Problem

At the top level (no parent rule), a & inside a functional pseudo-class is substituted with an empty string, producing invalid CSS:

serialize(compile(':is(&, .c){color:red}'), stringify)
// actual:   ":is(, .c){color:red;}"   <- leading comma, invalid
// expected: ":is(.c){color:red;}"

':is(&){color:red}'         -> ":is(){color:red;}"          // empty pseudo
':is(.c, &){color:red}'     -> ":is(.c, ){color:red;}"      // trailing comma
':is(.a, &, .b){color:red}' -> ":is(.a, , .b){color:red;}"  // empty middle arg

css-tree rejects the comma forms with Selector is expected:

import * as csstree from 'css-tree'
csstree.parse(':is(, .c){color:red}') // throws: Selector is expected

For comparison, the same input under a real parent already works, and a top-level & .x already compiles to a clean .x.

Root cause

In ruleset() the parent list at root is [''], so rule[x] === ''. The substitution replace(y, /&\f/g, rule[x]) turns the &\f marker into an empty string, so :is(&\f, .c) becomes :is(, .c). trim() only strips outer whitespace, so the empty argument and its comma stay inside the parentheses and the output is invalid.

Fix

When the parent is empty (root), remove the &\f marker together with one adjacent comma instead of leaving an empty argument. The non-root substitution path is unchanged.

-if (z = trim(j > 0 ? rule[x] + ' ' + y : replace(y, /&\f/g, rule[x])))
+if (z = trim(j > 0 ? rule[x] + ' ' + y : rule[x] ? replace(y, /&\f/g, rule[x]) : replace(y, /&\f *, *| *, *&\f|&\f/g, '')))

This mirrors the existing rule that a root & is removed (issue #333, and the "& root should be removed" test). The empty argument is dropped, so:

:is(&, .c)     -> :is(.c)
:is(.c, &)     -> :is(.c)
:is(.a, &, .b) -> :is(.a, .b)
:is(&)         -> :is()
:where(&)      -> :where()
:not(&)        -> :not()

All outputs parse cleanly with css-tree. Empty :is() / :where() / :not() are valid (forgiving selector list) and are the closest valid result when the only argument was the absent parent.

Tests

Added & root inside functional pseudo should be removed next to the existing root-& tests. It uses compile/serialize directly (no .user wrapper) so the empty-parent path is exercised.

• Red before the fix: expected ':is(, .c){color:red;}' to equal ':is(.c){color:red;}'.
• Green after the fix.
• Full suite: 114 passing, 0 failing (113 existing + 1 new). Lint and build pass.