Detect zig zon files

[softprops]

Add support for Zig package manager file extension .zig.zon files introduced in 0.11.0. Care was taken to be precise with the package managers file extension conventions, because I also found non-zig .zon files when doing a code search for just .zon files.

Description

Zig .zon files were introduced zig's 0.11.0 release to fulfill the zig build tool's declarative package management story for describing a project and its dependencies. Additional documentation on the format can be found here. It's syntax is valid zig, (essentially a these are struct files) so these files may be parsed and syntax highlighted the same as regular zig files. Today these have no syntax highlighting.

Checklist:

• I am adding a new extension to a language.
  • The new extension 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*.zig.zon
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/softprops/zig-envy/blob/main/build.zig.zon
    • Sample license(s): MIT
  • I have included a change to the heuristics to distinguish my language from others using the same extension.
    • note the .zig.zon qualification

[softprops]

@lildude sorry to bother you, but is there any eta on code review for pull requests?

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.