KQL Parsing support different 'flavors' of language when parsing

[albertlockett]

In #1722 we added support for an "if/else" type statement to the KQL, and there are more custom extensions we'd like to add to the grammar.

However, there might be some contexts where we're parsing the query for execution on a query-engine that doesn't support the statement. For example, record-set engine does not support the ConditionalDataExpression that was added in #1684

Ideally, we would have multiple flavors of the pest grammar, that can share the same base but extend it with additional syntax.

[albertlockett]

See https://pest.rs/book/grammars/grammars.html#load-multiple-grammars

[albertlockett]

closing as won't do. See #1734 (comment)