fix(mac): apply backspace to correct location 🍎

[sgschantz]

When applying a backspace or delete by executing an insert with a replacement range, calculate the correct location for the insert to avoid deleting the incorrect characters.

Build-bot: release:mac

Fixes: #16440
Fixes: #16448

User Testing

Preparation: install the keyboard bksp_ldml.zip
All tests should be using compliant apps such as Apple's Pages, TextEdit or Stickies.

• TEST_BKSP_D_ACUTE_VERIFY_CONTEXT: In a compliant app, with the BKSP keyboard selected, type:
  abc     d/
  Select the text, copy and paste it into a character viewer and verify that the last two characters are U+0064 U+0301.
• TEST_BKSP_D_ACUTE: In a compliant app, with the BKSP keyboard selected, type:
  abc     d/
  Then press bksp. The result should be abc d.
• TEST_EUROLATIN_E_ACUTE: In a compliant app, with the EuroLatin keyboard selected, type:
  caf'e
  The result should be café.
  Then press bksp. The result should be caf.
• TEST_EUROLATIN_BACKSPACE: In a compliant app, with the EuroLatin keyboard selected, type:
  abcreturnab
  Then press bksp four times. The result should be ab.
• TEST_REPLACE_SURROGATE_PAIR_CHARACTER: processing a character that causes the replacement of a surrogate pair produces the correct, uncorrupted output

1. Open the Stickies app and create a new Note
2. Switch to the Malar Tirhuta keyboard
3. Type k
4. Confirm that the letter 𑒏𑓂 was typed
5. Type e
6. Confirm that output now reads: 𑒏𑒹
7. Confirm that the text can be selected and successfully copied into a TextEdit document
8. Confirm that backspace deletes the letters in the order they were entered

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ⬜ TEST_BKSP_D_ACUTE_VERIFY_CONTEXT (OPEN)
• ⬜ TEST_BKSP_D_ACUTE (OPEN)
• ⬜ TEST_EUROLATIN_E_ACUTE (OPEN)
• ⬜ TEST_EUROLATIN_BACKSPACE (OPEN)
• ⬜ TEST_REPLACE_SURROGATE_PAIR_CHARACTER (OPEN)

Results Template

# Test Results

* **TEST_BKSP_D_ACUTE_VERIFY_CONTEXT (OPEN):** notes
* **TEST_BKSP_D_ACUTE (OPEN):** notes
* **TEST_EUROLATIN_E_ACUTE (OPEN):** notes
* **TEST_EUROLATIN_BACKSPACE (OPEN):** notes
* **TEST_REPLACE_SURROGATE_PAIR_CHARACTER (OPEN):** notes