feat(examples): add linting to all examples

[mafredri]

This PR adds linting and validation improved validation for examples templates and matches the requirements imposed by registry.coder.com.

The improvements caught an additional issue:

❯ make lint/examples
error: parse failed:
    main.run
        /Users/maf/Work/Code/coder/scripts/examplegen/main.go:87
  - example "templates/incus" has errors
front matter validation failed
icon does not exist: stat examples/templates/incus/icon/lxc.svg: no such file or directory
icon is not in site/static: "/icon/lxc.svg"
exit status 1

Ultimately the icon path was correct (as seen from the dashboard), but all other templates use a relative path so that we can verify the icons existence.

Here's another example after fudging up some more files:

❯ make lint/examples
error: parse failed:
    main.run
        /Users/maf/Work/Code/coder/scripts/examplegen/main.go:87
  - example "templates/gcp-devcontainer" has errors
front matter validation failed
tags isn't a slice: type bool
example "templates/gcp-linux" has errors
front matter validation failed
icon does not exist: stat site/static/icon/asdf.svg: no such file or directory
example "templates/incus" has errors
front matter validation failed
front matter does not contain "display_name"
front matter does not contain "maintainer_github"
icon does not exist: stat examples/templates/incus/icon/lxc.svg: no such file or directory
exit status 1

Fixes #12588

[johnstcn]

😅

[mafredri]

To be fair, both forms work. But all our other examples/templates use this relative form, and it's handy for confirming that the exact path exists.

[johnstcn]

Yep, +1 for enforced consistency!