Parent-Task:

• SR-10535 [stdlib] Drop ICU dependency

Issue Description:

ICU's grapheme break iterators have been a continual source of complexity and slowness in Swift. While we have fast-paths to work around them, Swift would benefit greatly from a native implementation driven from Unicode's data tables.

This would be an implementation behind a resilience barrier that we'd be tasked with updating for each new version of Unicode.

This would also be the biggest piece needed to wean Swift off of ICU as a dependency, which would be useful for Swift adoption in low-level systems, especially security-conscious ones.