Graduate manual_string_new to clippy::style

[tristan-f-r]

manual_string_new is a rule that standardizes how empty strings are created, which is not only a good style option (with no known false-positives in the docs), but also helps prevent against zero-width characters being submitted internally.

The Pull Request (#9366) that moved this to pedantic referred to it as:

• a fairly minor style change
• could be very noisy

However, not only are many existing non-pedantic style lints noisy (as they can appear a lot), but this one is also idiomatic and uses an in-built method to create an empty string.