feat(non-space-content): switch all non-empty checks to new generic check

[straker]

non-empty-alt, non-empty-title, and non-empty-value were all the same function so combined them into a single generic check. Generic checks are new checks that don't have metadata files associated with them so are just evaluate functions called from other checks (can't be used directly in a rule). They typically will need options passed into them to work.

I talked to wilco about non-empty-title using titleText function and he said it didn't need to do that so was fine to just look at the title attribute directly.

Lastly, all the tests for the old checks are the exact same, especially since now they use the same evaluate underneath. I think we should remove them and just have a single test for the generic one and let the integration tests for the rules that use these checks catch things, but we should discuss that here.

Closes issue: #2192

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