Track Parens<T>'s span

[xitep]

• this suggests Parens<T>. hopefully advancing [EPIC] Complete Span (source location) information / feature #1548 a bit and being the foundation for Inconsistent spans for function calls #2050
• it is (a bit) wasteful to store the parens' location as a Span (since a paren is just a single character, the start location would fully suffice); i simply decided for consistency with the rest of the AST.
• with this PR (so far), it's used only by Value so that i can properly determine the end of an insert statement and distinguish between "... VALUES ('foo' /\*xyz\*/) and ... VALUES('foo') /\*xyz\*/
• of course, many more places in the AST could be covered, e.g. function params/args, subqueries, etc. if this PR is OK as is, i can put in some more time and expand on the coverage. (just let me know.)