Move first_comment_regex to instructions extractor

What does this MR do and why?

• first_comment_regex method is used for parsing instruction, it fits into InstructionsExtractor class, instead of having it in TaskFactory and passing it as an additional parameter.
• moves also "intent" logic into instruction extractor (if intent is set we want to take it into account for detecting generation/completion)
• removes some duplicated tests from TaskFactory (as InstructionsExtractor is responsible for parsing prefix code it makes sense to cover all possible scenarios there)

Related to !134265 (comment 1608348539)

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