Validate: refactor internal methods to use `fs.FS` interface for file handling

[SBGoods]

Previously, the internal methods and checks for the validate command were reading files/directories from file paths, which made internal unit testing difficult as it required static files and directories for each test case. This made it difficult to test new schema concepts with confidence. This PR refactors the validateStaticDocs() and validateLegacyWebsite() to use a fs.FS interface for filesystem operations, and refactors the tests to use a fstest.MapFS as an in-memory filesystem along with additional integration tests for the various validation checks.

There were several bugs that came up during testing which have fixes in this PR:

• File extension check was only being run on index.md files instead of index.* files.
• File extension check was outputting the incorrect valid extensions it's error message.
• Frontmatter checks were not being run with the correct options on legacy index files.