Add Zig project discovery

[stephan-cr]

This PR adds Zig project discovery.

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by checking the relevant checkboxes):

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
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• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the docs (when adding new project types, configuration options, commands, etc)

Thanks!

[bbatsov]

Please, mention this in the changelog as well and in the docs (https://docs.projectile.mx/projectile/projects.html#file-markers)

[stephan-cr]

Please, mention this in the changelog as well and in the docs (https://docs.projectile.mx/projectile/projects.html#file-markers)

@bbatsov Done.

[bbatsov]

Thanks!