Use Diff Match Patch by default when available

[codeofdusk]

Link to issue number:

#3200.

Summary of the issue:

Diff Match Patch (DMP) was included but disabled by default in 2021.1. It fixes #3200 and allows for drastically fewer cross-process calls in UIA console, improving performance in busy applications.

Description of how this pull request fixes the issue:

This PR enables DMP by default to allow for wider user testing in master snapshots. If user feedback is positive, use by default in 2021.2.

Testing strategy:

Verified that DMP is used by default when feature flag is set to auto and Difflib when feature flag is set to force Difflib.

Known issues with pull request:

None.

Change log entries:

== Bug Fixes ==

• In terminal programs on Windows 10 version 1607 and later, when inserting or deleting characters in the middle of a line, the characters to the right of the caret are no longer read out. (better support for editing characters in the middle of lines in command prompt/terminal #3200)
  • Diff Match Patch now enabled by default. (Use Diff Match Patch by default when available #12485)

Code Review Checklist:

• Pull Request description is up to date.
• Unit tests.
• System (end to end) tests.
• Manual testing.
• User Documentation.
• Change log entry.
• Context sensitive help for GUI changes.
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers

[seanbudd]

Looks good to me, I have a suggestion for a docs change but the code looks good to me. Also pending approval from @Qchristensen

[Qchristensen]

userGuide change looks good - is there a change log entry? It's hiding from me.

[seanbudd]

@Qchristensen - we generally update the changelog manually just before merging. I've added this now, just need your approval to merge.

[Qchristensen]

UserGuide & Changes text looks good, thanks!