Improve doc comment code language tag parsing, don't use a full parser

[Alexendoo]

Now only tokenizes the contents of code blocks instead of spawning a thread for a full parser

The language tag handling now correctly picks up things like rust, ignore

no_test (#10491) is no longer specially handled, rustdoc considers rust,no_test to be a regular test

changelog: [needless_doctest_main], [test_attr_in_doctest]: now handles whitespace in language tags

[rustbot]

Some changes occurred in clippy_lints/src/doc

cc @notriddle

[rustbot]

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[github-actions]

Lintcheck changes for 0ff3a38

Lint	Added	Removed	Changed
clippy::needless_doctest_main	2	24	11
clippy::test_attr_in_doctest	2	0	1

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