Handle more edge cases when setting the user's cursor after auto-formatting an input

[seancolsen]

Reproduce

Note: I'm using the ￨ character here to denote a user's cursor -- not to represent an actual text character.

1. In Storybook, open the NumberInput component
2. Enter the number "1,000,000"
3. Place your cursor before the last zero: "1,000,00￨0"
4. Press Backspace, turning the input into "1,000,0￨0"
5. Expect the input to be reformatted as "100,00￨0"
6. Observe the input to be reformatted as "100,000￨" (the value is correct, but the cursor is in the wrong place)

Notes

• In Cursor position in FormattedInput #1257 we added a function getCursorPositionAfterReformat which determines the cursor position. Fixing this issue will require modifying the logic within that function.

• That function has a unit test with a number of test cases already written but commented out because they do not pass.

• The reproduction steps above correspond to the following test case:

  [  '1,000,00'  , 7                , '100,000'    , 6 ]