Rollup merge of #87596 - jesyspa:issue-87318-hidden-whitespace, r=estebank

Add warning when whitespace is not skipped after an escaped newline

Fixes issue #87318, also simplifies issue #87319.

* Add support to the lexer to emit warnings as well as errors.
* Emit a warning when a string literal contains an escaped newline, but when (some of) the whitespace on the next line is not skipped due to it being non-ASCII.