Fix composition mode

[offcall]

Summary

After this fix #2035
The composition mode began to break entities, this is because composedChars contains the text of the entire block (it causes unexpected behavior, since the entity text differs from its code), and not just the entered characters

My PR fixes this: gets the typed characters and inserts them in the right place

Tests

!need to replace old tests because they now depend on getSelection

[facebook-github-bot]

Hi @offcall!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at [email protected]. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[pro-vi]

Will this get merged?