Postgres 13 syntax support

[Alexey Borzov]

• New keywords: EXPRESSION, NFC, NFD, NFKC, NFKD, NORMALIZE, NORMALIZED, UESCAPE
• ONLY / WITH TIES option for SQL:2008 limit clause (FETCH FIRST...)
• IS NORMALIZED / IS NOT NORMALIZED operators
• NORMALIZE system function

NB: UESCAPE will be recognized as a keyword, but will not be processed until Lexer supports Unicode escapes