[gdb/cli] Allow source-highlight to autodetect language

[gdb/cli] Allow source-highlight to autodetect language

Currently when gdb asks the source-highlight library to highlight a file, it
tells it what language file to use.

For instance, if gdb learns from the debug info that the file is language_c,
the language file "c.lang" is used.  This mapping is hardcoded in
get_language_name.

However, if gdb doesn't know what language file to use, it falls back to using
python pygments, and in absence of that, unhighlighted source text.

In the case of python pygments, it autodetects which language to use based on
the file name.

Add the same capability when using the source-highlight library.

Tested on x86_64-linux.

Verified that it works by:
- making get_language_name return nullptr for language_c, and
- checking that source-highlight still manages to highlight a hello world.

Reviewed-By: Guinevere Larsen <blarsen@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>
PR cli/30966
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30966