Add OMNeT++ MSG language

[adamgeorge309]

This adds the OMNeT++ MSG language.

Description

MSG (extension .msg) is a DSL for the OMNeT++ network simulator. It is used by model frameworks for OMNeT++, such as the INET framework, Simu5G, Veins, etc. We are the team that develops OMNeT++ and the INET Framework and we have created a TextMate grammar for this language.

Checklist:

• [x] I am adding a new language.
  • [x] 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*.msg+opensim
  • [x] I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/inet-framework/inet
    • Sample license:
      • LGPLv3
  • [x] I have included a syntax highlighting grammar: https://github.com/omnetpp/omnetpp-textmate-msg
  • [x] I have added a color
    • Hex value: #a0e0a0
    • Rationale: Matches the branding guidelines for the OMNeT++ project.
  • [x] I have updated the heuristics to distinguish my language from others using the same extension.