Add codefix to make non-reassignable variables reassignable

[muradkhan101]

Adds a codefix for the scenarios where a user attempts to re-assign a variable that was declared as const or property that was declared readonly. In the former, it changes the initial declaration to type let and in the latter, it removes the readonly modifier.

Added one more diagnostic message as well since, I couldn't find one that fit well enough for the fix all option.

Fixes #22473

[muradkhan101]

Hmmm, anyone have a good idea about why the tests would fail on Travis but succeed on my local machine?

Edit: Found out I forgot to include my codefix file in the tsconfig

[muradkhan101]

@DanielRosenwasser I updated the PR and implemented the changes you recommended. Thanks for the info btw, it really helped to clean up the logic and simplify it a lot!