Fix greedy lookbehinds with sticky regexes

The problem

Right now, greedy patterns with lookbehinds have a problem: the lookbehinds don't work properly.

Here's an example:

JavaScript (and many other C-like languages) use(s) the C-like comment patterns:

Prism.languages.clike = {
	'comment': [
		{
			pattern: /(^|[^\\])\/\*[\s\S]*?(?:\*\/|$)/,
			lookbehind: true,
			greedy: true
		},
		...

Why does this bug occur? It's actually pretty simple. Let's step through our matching algorithm!

1. Let text be the string "/**//**/" and let pattern be the C-like comment regex.
2. First we set pattern.lastIndex = 0.
3. Let m be pattern.exec(text). m is now the match ["/**/", "", index: 0]. The lookbehind group is empty because it accepted using the ^ assertion. pattern.lastIndex is now 4.
4. Create the token for the first /**/. Then we get ready for the next iteration.
5. Let m be pattern.exec(text). m is null now. The pattern rejected text because of the lookbehind group. The ^ alternative obviously rejects because the second /**/ isn't at the start of the string. The [^\\] alternative accepts but leads nowhere because it consumes the first / of the second /**/.
6. The matching algorithm moves on to other patterns.

The reason why the second line in the example gets highlighted correctly is the space between the two comments. The [^\\] alternative can consume the space and so the comment can be detected.

I highlighted the problem with this simple example but this is a bit more general. All greedy patterns with lookbehinds are affected.

Note: Non-greedy patterns are not affected by this problem at all because the matching algorithm works differently for them.

The solution

We can fix this with the ES6 sticky y flag.

Here is the fixed version of the above example:

Prism.languages.clike = {
	'comment': [
		{
			pattern: /(|[\s\S]*?[^\\])\/\*[\s\S]*?(?:\*\/|$)/y,
			lookbehind: true,
			greedy: true
		},
		...

I haven't performance-tested this yet, so it might be slower.

An even better solution would be to use ES2018 lookbehind assertions. See #1708 as for why this isn't possible right now.

Start with the C-like comment definition in components/prism-clike.js and reproduce the linked test case at prismjs.com/test.html. Trace the matching algorithm for greedy patterns with lookbehinds, then verify that adjacent C-like comments are highlighted correctly without breaking the existing behavior.