It's not used, probably because it's pretty strange and hard to imagine using
safely. (Stripping leading and trailing space could be useful, but collapsing
sequences of spaces? Hmm.)

This commit does the following.

• Renames slashslash as dumbComments. As a result, it now comes before
  emptyLines in alphabetical ordering, which means that if you apply both
  dumbComments and emptyLines, lines that contain only comments will be
  fully removed.

  (I contemplated changing the filter ordering to match the order specified,
  rather than using alphabetical ordering, but that was more invasive and not
  obviously better.)

• Changes dumbComments so it only applies if the comment is at the start of
  the line (with optional leading whitespace). This is so it can be used with
  prefs files, which contain lines like pref("foo", "https://mozilla.org");
  where the // must not be treated as a comment.

Note that slashslash wasn't being used anywhere.

Depends on D88240

For a local build on my Linux64 box, this reduces greprefs.js from 174,906
bytes to 103,221 bytes, and firefox.js from 112,100 bytes to 68,656 bytes.

Depends on D88242

https://hg.mozilla.org/mozilla-central/rev/76efaf59feb8
https://hg.mozilla.org/mozilla-central/rev/bcc9e11ca06b
https://hg.mozilla.org/mozilla-central/rev/328067bcffe8