fix(issue:4267) support starting-style at-rule

[puckowski]

What:

This pull request resolves issue #4247 by fixing @starting-style at-rule nesting.

Why:

Users of Less.js may wish to use newer features of CSS supported by newer versions of most browsers. Estimated support for @starting-style is 84.76%+ (https://caniuse.com/mdn-css_at-rules_starting-style).

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

Bundle size:

Less.js 4.2.1 minified is 151,936 bytes.
With PR merged Less.js bundle minified becomes 155,088 bytes.
Delta: 3,152 bytes

Lines of code delta: 275 added (including tests)

The following Less:

[popover]:popover-open {
    opacity: 1;
    transform: scaleX(1);

    @starting-style {
        opacity: 0;
        transform: scaleX(0);
    }
}

becomes:

[popover]:popover-open {
  opacity: 1;
  transform: scaleX(1);
  @starting-style {
    opacity: 0;
    transform: scaleX(0);
  }
}

[puckowski]

I submitted a few PRs. I expect rebase(s) will be needed in order to merge all of them, so let me know when a rebase is needed.