Validation for Embedded SQL Strings + Formatter Improvements

Added

• parser: validation for sql embedded in strings (#1350)

• parser: validation for uescape (#1349)

• parser: plpgsql support (#1348, #1347, #1346, #1345)

• playground: semantic syntax highlighting for formatted output (#1344)

Changed

• fmt: improve alter table, fk, do, case when else, @ op (#1354)
• fmt: support nicer single expression clauses (#1343)