feat(core): normalize input context

First half of #9999.

Adds support for normalization (to NFD) of input app context into the
cached context. The keyboard processor will work with the NFD cached
context.

Adds unit tests for the normalization as part of the LDML keyboard
processor test suite.

TODO:
* Comparing modified cached context to app context to determine the
  transform required to send to the app
* Handling illegal unicode and unpaired surrogates on input context