Add FlatBuffers language support

[aidalgol]

Description

Add the FlatBuffers schema language.

Checklist:

• 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:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.fbs+flatbuffers
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/google/flatbuffers/blob/master/reflection/reflection.fbs
      • https://github.com/apache/arrow/blob/main/format/Message.fbs
    • Sample license(s): Apache 2.0
  • I have included a syntax highlighting grammar: https://github.com/floxay/vscode-flatbuffers
  • I have added a color
    • Hex value: #ED284A
    • Rationale: This is the main color in the FlatBuffers logo. https://github.com/google/flatbuffers
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Important

The changes in this PR will not appear on GitHub until the next release has been made and deployed. See here for more details.