fix(type): handle selectionRange properly

[kentcdodds]

What: fix(type): handle selectionRange properly

Why: Closes #309

How: create a new calculateNewBackspaceValue out of the calculateNewValue function because there's no reasonable way to abstract that. I promise.

Checklist:

• Documentation N/A
• Tests
• Typings N/A
• Ready to be merged

[codecov]

Codecov Report

Merging #313 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
+ Coverage   99.26%   99.31%   +0.05%     
==========================================
  Files           2        2              
  Lines         273      293      +20     
  Branches       70       74       +4     
==========================================
+ Hits          271      291      +20     
  Misses          2        2              

Impacted Files	Coverage Δ
src/type.js	100.00% <100.00%> (ø)

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[kentcdodds]

🎉 This PR is included in version 11.2.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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