Incorrect syntax highlighting for .zshrc (BB-11605)

I'm not sure what syntax is actually being used, but a file named .zshrc should be highlighted like a shell file, and it's instead highlighting dollar signs in red as errors.

Example here: https://bitbucket.org/kevinburke/small-dotfiles/src/6d1529e92f70717ed545b47f68a3400a6b4db762/.zshrc?at=master#cl-4