jam/ham highlighting

"It would be nice" if jam/ham highlighting support was introduced.

Here are some samples:

• https://github.com/boostorg/boost/blob/master/Jamroot
• https://github.com/boostorg/move/blob/70623d0215da773538e004ef515b48fd76dc5856/Jamfile
• https://git.haiku-os.org/haiku/tree/Jamfile
• https://git.haiku-os.org/haiku/tree/src/system/Jamfile

Key parts:

• "*.jam" is a good detection for Jam/Ham files
• "Jamfile,Jamroot" are standard names for Jam files (aka like Makefile)
• Overall syntax is Makefileish, but with some differences