Auto merge of #17855 - asajeffrey:script-implement-more-stylepropertymaypreadonly, r=emilio

Implement getProperties for StylePropertyMapReadOnly

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Implement `getProperties` for `StylePropertyMapReadOnly`.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17579.
- [X] These changes do not require tests because the existing css-paint-api tests catch this (rather annoyingly, they all fail for different reasons, sigh)

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tests/wpt/mozilla/meta/mozilla/css-paint-api/style-background-image.html.ini

@@ -1,4 +1,4 @@
 [style-background-image.html]
   type: reftest
   expected: FAIL
-  bug: https://github.com/servo/servo/issues/17579
+  bug: https://github.com/servo/servo/issues/17378

tests/wpt/mozilla/meta/mozilla/css-paint-api/style-before-pseudo.html.ini

@@ -1,4 +1,4 @@
 [style-before-pseudo.html]
   type: reftest
   expected: FAIL
-  bug: https://github.com/servo/servo/issues/17579
+  bug: https://github.com/servo/servo/issues/17854

tests/wpt/mozilla/meta/mozilla/css-paint-api/style-first-letter-pseudo.html.ini

@@ -1,4 +1,4 @@
 [style-first-letter-pseudo.html]
   type: reftest
   expected: FAIL
-  bug: https://github.com/servo/servo/issues/17579
+  bug: https://github.com/servo/servo/issues/17854