Add MCNP syntax

[deathaxe]

• I'm the package's author and/or maintainer.
• I have have read the docs.
• I have tagged a release with a semver version number.
• My package repo has a description and a README describing what it's for and how to use it.
• My package doesn't add context menu entries. *
• My package doesn't add key bindings. **
• Any commands are available via the command palette.
• Preferences and keybindings (if any) are listed in the menu and the command palette, and open in split view.
• If my package is a syntax it doesn't also add a color scheme. ***
• If my package is a syntax it is named after the language it supports (without suffixes like "syntax" or "highlighting").
• I use .gitattributes to exclude files from the package: images, test files, sublime-project/workspace.

My package provides MCNP syntax definition and snippets for ST4.

Syntax is primarily sourced from https://github.com/repositony/vscode_mcnp and has been converted to sublime-syntax format with some optimizations added.

[packagecontrol-bot]

Automated testing result: SUCCESS

Repo link: MCNP

Packages added:
  - MCNP

Processing package "MCNP"
  - All checks passed

[FichteFoll]

Happened to look at this already briefly when it was pushed (since I get emails for that), so this was a quick review.