Add priority to createGenericPropertyDescriptor()

[asamuzaK]

Fix for jsdom/jsdom#3940

Also with

• Fixed a potential bug and refactored parsePropertyValue() handling in parsers.js
• Dependencies and devDependencies update

I noticed that we can do above when trying to install cssstyle@6.0.0 in jsdom.

[domenic]

I wish this were separated into smaller PRs, but oh well.

[domenic]

Oops, I clicked the merge button without even editing the commit message. Oh well.

If you have suggestions for release notes on the "potential bug", let me know and I will add them to https://github.com/jsdom/cssstyle/releases/tag/v6.0.1.

[asamuzaK]

Bug fix is:

• Correctly return case-sensitive value within CSS function. e.g. path("m 5,5 h 35 L 20,30 z")