MDOCS-3155 soft line break bug fix

[simon-d-miro]

Motivation

When pasting text with a soft break in it near the end of a line, the soft break would (1) not be inserted and (2) the orignal preceeding characters would replace the soft breaks. For example

Document:

0123
   ^ - insert at index 3

Change Delta:
{insert: '45[SOFT_LINE_BREAK]67'}

Invalid outcome produced
01245367

Investigation

The issue is in block.ts

 softLines.forEach((str) => {
        if (i < this.length() - 1 || this.children.tail == null) {

After the first invocation of the loop, we should go into the first if statement but we don't because
i < this.length() - 1
doesn't take into account the fact we have modified the document

Fix

1. Check the if statement first
2. update insertIndex calculation in the first if Math.min(i, this.length() - 1 + addedChars);

[DeanWay]

You can apply these suggestions if you like but otherwise the solution works as is, LGTM