Add Tiltfile filename to Starlark language

[jazzdan]

This PR adds syntax support for Tiltfiles which are written in Starlark.

Checklist:

• I am associating a language with a new file extension.

  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?l=&q=filename%3ATiltfile&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/windmilleng/tilt/blob/master/integration/live_update_two_images_one_manifest/Tiltfile
    • Sample license(s): Apache 2.0
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am adding a new language.

  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a syntax highlighting grammar: https://github-lightshow.herokuapp.com/
  • I have updated the heuristics to distinguish my language from others using the same extension.

• I am fixing a misclassified language

  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am changing the source of a syntax highlighting grammar

  • Old: https://github-lightshow.herokuapp.com/
  • New: https://github-lightshow.herokuapp.com/

• I am updating a grammar submodule

• I am adding new or changing current functionality

  • I have added or updated the tests for the new or changed functionality.

• I am changing the color associated with a language

  • I have obtained agreement from the wider language community on this color change.
    • [URL to public discussion]
    • [Optional: URL to official branding guidelines for the language]

[pchaigno]

Looks like these changes were introduced by mistake.

[jazzdan]

Whoops, thanks for pointing that out @pchaigno. Should be fixed now