refactor: replace insertText from #6553 changes to reflect new removeText feature

[vantage-ola]

This PR aims to Refactor #6553 : simplified insertText rewrite (part 2)

Right now, without changing the commented code that uses the new removeText feature. It passes almost all the tests except 2 cases

● LexicalSelectionHelpers tests › Collapsed › Can handle a text point

   expect(received).toBe(expected) // Object.is equality

   Expected: "Testa"
   Received: "a"

     148 |         selection.insertText('Test');
     149 |
   > 150 |         expect($getNodeByKey('a')!.getTextContent()).toBe('Testa');
         |                                                      ^
     151 |
     152 |         expect(selection.anchor).toEqual(
     153 |           expect.objectContaining({

 ● LexicalSelectionHelpers tests › Simple range › Can handle multiple text points

   expect(received).toBe(expected) // Object.is equality

   Expected: "Test"
   Received: ""

     1352 |         selection.insertText('Test');
     1353 |
   > 1354 |         expect($getNodeByKey('a')!.getTextContent()).toBe('Test');
          |                                                      ^
     1355 |
     1356 |         expect(selection.anchor).toEqual(
     1357 |           expect.objectContaining({

I will be investigating why this is the issue and as @GermanJablo said, i will make some refinements to his draft insertText to fix those errors