refactor: Inline stable IMAP extensions.

[coveralls]

Pull Request Test Coverage Report for Build 5880992448

• 11 of 12 (91.67%) changed or added relevant lines in 5 files are covered.
• 11 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.07%) to 93.498%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-codec/src/command.rs	4	5	80.0%
<!--	Total:	11	12	91.67%	-->

Files with Coverage Reduction	New Missed Lines	%
imap-codec/src/codec/decode.rs	1	96.3%
imap-codec/src/codec/encode.rs	2	91.58%
imap-codec/src/command.rs	2	94.95%
imap-types/src/core.rs	2	92.53%
imap-types/src/response.rs	4	69.25%
<!--	Total:	11		-->

Totals
Change from base Build 5880503808:	-0.07%
Covered Lines:	8729
Relevant Lines:	9336

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💛 - Coveralls