Pattern Validation: Do some content validation early

[ryelle]

PR https://github.com/WordPress/pattern-directory/pull/477 introduced some content validation on the PHP side, but it would be nice if we could validate the pattern content before sending someone through the whole submission modal.

From https://github.com/WordPress/pattern-directory/pull/477#issuecomment-1100960512

Could we run the < 3 & > 75 checks on clicking Submit and show the red notice as is? That would make the creator align more with the way Gutenberg handles content/API errors. The title validation could be done inline where the input is in the modal... before going through the hassle of writing a description and choosing categories just to have it fail at something that was technically step 1.