refactor: Rename `Continue` to `CommandContinuationRequest`.

[coveralls]

Pull Request Test Coverage Report for Build 5799345246

• 12 of 15 (80.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 93.399%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-codec/src/codec/encode.rs	3	4	75.0%
imap-codec/src/response.rs	3	4	75.0%
imap-types/src/response.rs	5	6	83.33%
<!--	Total:	12	15	80.0%	-->

Totals
Change from base Build 5798779361:	0.02%
Covered Lines:	8772
Relevant Lines:	9392

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💛 - Coveralls