https://github.com/kaushalmodi/hello_musl/blob/master/config.nims
IIUC, syntax highlighting on github is handled in https://github.com/github/linguist and yes they accept PR's
https://github.com/nim-lang/Nim/blob/devel/tests/deps/zip-0.2.1/zip.nimble
https://github.com/nim-lang/Nim/blob/devel/config/nim.cfg (nit: some constructs aren't highlighted, but ok)
eg: tests/deps/zip-0.2.1/zip.nimble
eg: tests/test_nimscript.nims
config/nim.cfg
reported in https://github.com/Varriount/NimLime/issues/108