Add JAR Manifest file

[Nixinova]

• Resolve Add support for MANIFEST.MF #5503

Description

Adds support for JAR MANIFEST.MF files.

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:
      • manifest.mf: 1.7M
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • MANIFEST.MF: GPL-2 (but autogenerated)
  • I have included a syntax highlighting grammar:
    • Uses YAML's one. Lightshow doesn't like it but Manifest is valid YAML syntax so there shouldnt be an issue. Gist works though.
  • I have updated the heuristics to distinguish my language from others using the same extension.

[Nixinova]

Why the case sensitivity?

  1) Failure:
TestSamples#test_filename_listed [/home/runner/work/linguist/linguist/test/test_samples.rb:49]:
/home/runner/work/linguist/linguist/samples/Manifest/filenames/MANIFEST.MF isn't listed as a filename for Manifest in languages.yml.
Expected ["manifest.mf"] to include "MANIFEST.MF".

  2) Failure:
TestFileBlob#test_language [/home/runner/work/linguist/linguist/test/test_file_blob.rb:652]:
No language for /home/runner/work/linguist/linguist/samples/Manifest/filenames/MANIFEST.MF