Keep all parent selectors inside :not()

[wahidrizka]

As discussed in #176.

With a selector list as the parent, & inside :not() was replaced with one parent selector per rule, which turns "neither .a nor .b" into "not .a, or not .b":

.a, .b {
  .x:not(&) { color: red }
}

/* before */
.x:not(.a), .x:not(.b) { color: red }

/* after, like Sass 1.105.1 */
.x:not(.a, .b) { color: red }

Now & inside :not() gets the whole parent list in its own selector list, also inside :is(), :where() or :has() within :not(), and several & there take every combination, like top-level & since #176. Outside :not() nothing changes, so .x:is(&) still gives .x:is(.a), .x:is(.b). :not(:nth-child(2n of &)) also keeps the old output, because the part after of is not a plain selector list.

To check it, I compared the output with Sass on distinct generated nested rules that use & inside :not() in different ways. On main, 1,333 of 2,699 rules gave a different set of selectors, now none do (another seed: 1,324 of 2,651, now none). On 4,000 more distinct general rules, 143 more now match Sass and none stopped matching. In Chrome 154, the output now styles the same elements as native nesting in all 6 cases I tried, main differed in 5.

(Edited: my first numbers counted some generated rules twice. These count each distinct rule once.)

The new test fails on main. bnt --coverage 100, oxlint and oxfmt --check pass.