Add Procfile language

[Nixinova]

• Resolves Proposal: Procfile language #5593

Description

Adds the Procfile filename/language used in Heroku web apps.

Colour #3B2F63 from topbar in Heroku docs

Type is programming, but data might be better? It's a list of commands of format name: cmd with --args.

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:
      • 600K+ in repo roots, 770K+ total
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • postalserver/postal
    • Sample license(s):
      • MIT
  • I have included a syntax highlighting grammar:
    • Source | License
    • Preview
  • I have updated the heuristics to distinguish my language from others using the same extension.

[blockarchitech]

Looks good!

Procfile is indeed used in thousands of repositories across GitHub, so I don't see why this wouldn't be added.

[Alhadis]

Type is programming, but data might be better? It's a list of commands of format name: cmd with --args.

Makefiles are composed of similar declarations, and since we categorise those as programming, I'd say the current designation is appropriate. 👍