[feature](istanbul-lib-instrument): update ignore regex for "legal" comments

[kindoflew]

Resolves #658.

This PR adds support for what esbuild calls "legal comments" (https://esbuild.github.io/api/#legal-comments).

The issue being that esbuild strips comments when building, so any /* istanbul ignore (if | else | next) */ comments are ignored when using it (I recently ran into this exact issue trying to get test coverage with a vite project). However, esbuild will leave "legal" comments in (ie, if the comment starts with /*! or //!).

Caveats

According to the above-linked section in esbuild's docs, this only applies to "statement-level" "legal" comments:

Note that "statement-level" for JS and "rule-level" for CSS means the comment must appear in a context where multiple statements or rules are allowed such as in the top-level scope or in a statement or rule block. So comments inside expressions or at the declaration level are not considered license comments.

I interpret this to mean that the following example might not work:

const fooOrBar = baz ? 'foo' : /*! istanbul ignore next */ 'bar'; 

However, I don't believe there is anything istanbul can do to solve this, so I'm only noting it here for completeness.

Documentation (?)

Should we document this somewhere? esbuild is becoming a pretty popular tool and this issue may start to come up more often. I think a footnote in istanbul/nyc's README would probably be enough?

Side Note

I also discovered that there is inconsistent handling of RegEx matching between /* istanbul ignore (if | else | next) */ and /* istanbul ignore file */.

When the if | else | next RegEx is tested, it uses v.match(COMMENT_RE). But when file is tested, it uses COMMENT_FILE_RE.test(c.value), which is less strict (I altered one of the tests so the comment read /* plz istanbul ignore file */ and it still passed). I don't know if this is intentional or not, but I noticed it while implementing this and thought I'd bring it up in case it was an oversight.

Thanks for this rad library! Let me know if there is anything else I can do!

[9still]

@kindoflew @AriPerkkio any hope on getting this updated/merged?