Rename font-subset to font_subset.

[matanlurey]

For consistency, I don't think we have any other tools with -'s.

I'll be honest - this seemed easier than teaching the header guard tool how to handle -'s in directory names, but if you feel strongly about it I can revert.

[jason-simmons]

Changing the name of the source directory to font_subset for consistency makes sense.

But the binary name font-subset is expected by downstream consumers like flutter_tools and would require a coordinated update. I'd recommend keeping that as is.

[matanlurey]

But the binary name font-subset is expected by downstream consumers like flutter_tools and would require a coordinated update. I'd recommend keeping that as is.

Makes sense. Done. PTAL!

[matanlurey]

Oops I messed something up with this PR (merged weird or something). Fixing...