Adjust the code style according to the rules that the linting process in Gutenberg requires.
There are only a couple code changes that should have no effect on the runtime:
- A missing check to verify that only `UTF-8` is supported has been added (brought up because it was identified as an undefined variable).
- A few `return false;` statements have been added to avoid having the linter complain that functions don't return a value despite indicating they return `bool`. The functions are stubs for coming support and currently `throw`, so the `return` statements are unreachable.
Props dmsnell, costdev, davidbaumwald, peterwilsoncc, SergeyBiryukov.
Fixes#58918.
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