[JSX] Improve commenting behavior.

[Thom1729]

Should fix #2461.

Given the following:

<foo>
    bar!
</foo>;

The toggle_comment command will comment the second line as follows:

<foo>
    {/*bar!*/}
</foo>;

But when you call it again to uncomment, you get:

<foo>
    {bar!}
</foo>;

…instead of the original, which we want. This is due to internal implementation details of toggle_comment — it won't remove {/*/*/}, even though they're specified in JSX Comments.tmPreferences, because the comment scope doesn't extend to the braces.

This PR adds a special case to JSX interpolations so that when an interpolation contains only a single block comment, the entire interpolation including the braces will get the comment scope, so that the toggle_comment command will do what we want. This is not really the correct scoping, but the previous behavior is really annoying and we can't really change toggle_comment.

In addition, this PR improves the behavior of commenting attributes. E.g.:

<Foo
  bar={42}
/>

…will become:

<Foo
  // bar={42}
/>

[Thom1729]

I've simplified the comments file substantially, but they should behave the same.

Note that there are still some broken corner cases due to sublimehq/sublime_text#2152.