Moved lints from strings to methods

[MassimilianoBaglioni]

changelog: none

Moves STRING_FROM_UTF8_AS_BYTES and TRIM_SPLIT_WHITESPACE from strings.rs into the shared Methods dispatch in methods.rs.

Questions:

• TRIM_SPLIT_WHITESPACE now inherits the from_expansion() check in Methods::check_expr. It had no macro guard before. Tests still pass (no test covers macro-expanded call sites), but this is an untested behavior change worth flagging.
• I tried moving STRING_LIT_AS_BYTES too, but some tests use macros, and the from_expansion() check in Methods::check_expr prevents the lint from firing on those cases, so tests failed. I kept it in strings.rs for now because of this. How should I proceed?
• STR_TO_STRING not moved yet, one branch matches ExprKind::Path, not MethodCall, so it doesn't fit the shared dispatch. Should this get its own arm in check_expr, stay separate, or something else?

Also happy to get feedback/comments on the code already moved, not just on the open questions above.

fixes #14253

[Jarcho]

Can you split this into one commit per lint moved, please. Also when fixing CI errors you're better off force-pushing. We're going to ask you to squash those commits before merging and it doesn't affect reviewers much if you're immediately changing commits.