Update parser grammar to match RFC3501 and RFC4466

[nevans]

• parse most responses in the wild, generally correctly 🙂
• Capability in response code #6 ATOM-CHAR vs ASTRING-CHAR
• Capability in response code #6 CAPABILITY from response code
• Fix text_response and resp-text #8 resp-text may begin with "[" or "=""]"
• add BADCHARSET support #9 add BADCHARSET response code (Errata Exist)
• Complete IMAP4rev1 ABNF (Errata Exist)
• RFC4466 (2006): Collected Extensions to IMAP4 ABNF #35
• ✨🐛 Update BODYSTRUCTURE parser; add location; fix bugs #113