Account for titlecase in casing lints

[Jules-Bertholet]

Puts #153892 to work.

Also contains fixes for Greek final sigma casing.

There are probably still some edge cases left to fix. Ideally we would use https://www.unicode.org/reports/tr55/#Identifier-Chunks as a base.

@rustbot label A-Unicode A-diagnostics A-lints A-suggestion-diagnostics

[rustbot]

r? @petrochenkov

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