Improve validation of `URLPathTemplate` and `JSONSelection` strings passed to `@source{Type,Field}` directives

[benjamn]

Follow-up to #2910.

This work is not finished because we ultimately want to validate that the variables used by URLPathTemplate strings and the output shapes of JSONSelection strings match the available field and argument names.

This PR adds parsers for the two syntaxes and tests of those parsers, which allows us to validate the strings have the expected syntax, even if we're not validating semantics yet.

As another TODO, when we use JSONSelection for body parameters, it's the input shape of the selection that needs to be validated, rather than the output shape, so getSelectionOutputShape may not be sufficient.

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