add FreeFem++ lexer

[sgarnotel]

Lexer for FreeFem++ (Github repo).

This is a finite element language, C++ idiom but with a lot of difference (so, new class)

Thanks in advance for merging!

[franckl]

👍 it would be very useful to have. Looking forward to it !

[pyrmont]

@sgarnotel I'm sorry it's taken so long to get to this PR. Thank you for submitting it!

After going over it, I wondered if, given the similarities between this lexer and the C lexer (unsurprisingly given the C++ idiom), it wouldn't be preferable to inherit from the C (or perhaps the C++) lexer class and then extend where necessary?

That would greatly cut down on the code in this lexer, hopefully making it easier to review and maintain. You can see an example of how this would work with the C++ lexer.

[sgarnotel]

Thanks for your reply, I opened a new PR for the inherited lexer
#1356