Add and/or query validation

Created by: rvantonder

The and/or parser introduces a new way to parse parameters and search patterns. We need to validate general properties:

• Valid regular expression values for filters like repo:...
• Valid values for filters like case:...

Progress for this part is tracked in #9173 (closed).

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We also search-specific syntax validation for literal, regexp, structural search. Examples:

• Literal search is the absence of validation. We need to test that it is appropriately disambiguated from valid parameter as above, though.
• Regexp search patterns must compile.
• Structural search patterns should have balanced delimiters.

Progress for this part is tracked in #9174 (closed).

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Eventually, we want this validation of leaf expressions in and/or queries to be functionally equivalent to our current validation (but not necessarily using the same implementation).