JS: Improved contextual keywords

[RunDevelopment]

This improves how JS handles contextual keywords.

Changes:

• finally now looks for a { after instead of a } in front.
• from now looks for the start of a string after (this resolves Javascript syntax rendering error #2712).
• All contextual keywords will now default to being highlighted if no context is found. This kind of fallback is important because of Prism's matching algorithm. Basically, these contextual keywords use positive lookaheads to assert the context but that context may or may not be part of another token because of matching order and therefore may or may not be inaccessible to the contextual keywords. $ assertions have been added to all lookaheads to implement the fallback.

There might be a few other keywords that would also benefit from context assertions but I haven't seen them causing issues yet, so let's ignore them for now.

[github-actions]

JS File Size Changes (gzipped)

A total of 1 files have changed, with a combined diff of +8 B (+0.6%).

file	master	pull	size diff	% diff
components/prism-javascript.min.js	1.3 KB	1.31 KB	+8 B	+0.6%

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