script: Implement deprecated CSSStyleSheet members (#36313)

Implements `rules`, `addRule()` and `removeRule()` for `CSSStyleSheet`.
https://drafts.csswg.org/cssom/#legacy-css-style-sheet-members

This is part of #36162

Testing:
- `/css/css-cascade/at-scope-parsing.html`
- `/css/css-conditional/at-supports-whitespace.html`
- `/css/css-nesting/invalidation-004.html`
- `/css/css-nesting/parsing.html`
- `/css/css-nesting/serialize-group-rules-with-decls.html`
- `/css/css-syntax/custom-property-rule-ambiguity.html`
- `/css/css-syntax/invalid-nested-rules.html`
- `/css/css-syntax/trailing-braces.html`
- `/css/css-syntax/var-with-blocks.html`
- `/css/css-transitions/parsing/starting-style-parsing.html`
- `/css/cssom/CSSStyleSheet.html`
- `/css/cssom/idlharness.html`
- `/css/cssom/insertRule-across-context.html`

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

tests/wpt/meta/css/css-syntax/custom-property-rule-ambiguity.html.ini

@@ -1,12 +0,0 @@
-[custom-property-rule-ambiguity.html]
-  [Rule that looks like a custom property declaration is ignored]
-    expected: FAIL
-
-  [Rule that looks like an invalid custom property declaration is ignored]
-    expected: FAIL
-
-  [Nested rule that looks like a custom property declaration]
-    expected: FAIL
-
-  [Nested rule that looks like an invalid custom property declaration]
-    expected: FAIL

tests/wpt/meta/css/css-syntax/invalid-nested-rules.html.ini

@@ -1,3 +0,0 @@
-[invalid-nested-rules.html]
-  [Continues parsing after block on invalid rule error]
-    expected: FAIL

tests/wpt/meta/css/css-syntax/trailing-braces.html.ini

@@ -1,3 +0,0 @@
-[trailing-braces.html]
-  [Trailing braces are not valid]
-    expected: FAIL

tests/wpt/meta/css/css-syntax/var-with-blocks.html.ini

@@ -1,42 +0,0 @@
-[var-with-blocks.html]
-  [Plain var()]
-    expected: FAIL
-
-  [Whole-value block with var()]
-    expected: FAIL
-
-  [Whole-value block with var() (spaces)]
-    expected: FAIL
-
-  [Trailing block, leading var()]
-    expected: FAIL
-
-  [Leading block, trailing var()]
-    expected: FAIL
-
-  [In-block var() with trailing token]
-    expected: FAIL
-
-  [In-block var() with leading token]
-    expected: FAIL
-
-  [Plain var() (custom property)]
-    expected: FAIL
-
-  [Whole-value block with var() (custom property)]
-    expected: FAIL
-
-  [Whole-value block with var() (spaces, custom property)]
-    expected: FAIL
-
-  [Trailing block, leading var() (custom property)]
-    expected: FAIL
-
-  [Leading block, trailing var() (custom property)]
-    expected: FAIL
-
-  [In-block var() with trailing token (custom property)]
-    expected: FAIL
-
-  [In-block var() with leading token (custom property)]
-    expected: FAIL