Add the Scenic language

[dfremont]

Description

Adds a grammar and sample files for Scenic, a domain-specific probabilistic programming language for modeling the environments of cyber-physical systems (e.g. autonomous vehicles).

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Per FYI: Temporary change in language and extension popularity assessment #5756, I checked that there are >2,000 .scenic files on GitHub, under many different users and repos.
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3Ascenic+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • All samples are drawn from the Scenic repository; specific paths are given in each file.
    • Sample license(s):
      • The sample files (like the rest of the Scenic repo) are available under the 3-Clause BSD License. As the primary author of Scenic, I agree to distribute them in linguist under the MIT License.
  • I have included a syntax highlighting grammar: https://github.com/UCSCFormalMethods/Scenic-tmLanguage
  • I have updated the heuristics to distinguish my language from others using the same extension.
    • Not necessary as no other languages use the .scenic extension.