Hi, this PR adds a lexer for Gleam, which just released version 1.0.0.

I've run tests and things look good to me. However, I'm not a Python person and haven't tried writing a lexer before, particularly not one for Pygments, so I'm happy to take feedback and suggestions for improvements.

There is some example HTML at https://red-rose.com/gleam.html that I generated with pygmentize and then modified to add the lightbulb style (slightly modified for Name colours). This is the same large example file that's included with the PR.

Cheers,
Daren