Add STL support and enable STAR highlighting

[Alhadis]

This pull-request combines two unrelated changes that both depend on recent changes pushed to language-etc (so it would've been a hassle to split them into separate PRs).

Description

1. STL has been added as a recognised language.
   Technically, GitHub already supports .stl files in the form of an interactive viewer (both in the file-viewer and in fenced code-blocks). However, Linguist currently classifies these files as text. This PR adds STL as a first-class language, improving discoverability in code-search and enabling syntax highlighting in source view).
   
   The colour I chose, #373b5e, was averaged from a thumbnail of the cover for Automated Fabrication, which was the book that first documented the STL format.
   
   
2. Syntax highlighting is enabled for the recently-added STAR format.
   A grammar for STAR files was added today in Alhadis/language-etc@42e30c8.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories.
    For once, I didn't bother doing a code-search. I'm going on a limb and assuming that if GitHub went out of their way to add 3D viewer functionality specifically for .stl files (even when other formats are more deserving), then STL is probably plenty relevant enough already.
    
    
  • I have included a real-world usage sample.
    The various cube*.stl samples were all edited from the STL sample featured in GitHub's documentation.
    
    
  • I have included a change to the heuristics to distinguish my language from others using the same extension.
    Because .stl is a very common extension spanning a broad variety of uses, I've added it as a generic extension.
    
    
• I am changing the source of a syntax highlighting grammar
• I am updating a grammar submodule