fix(caret): trailing spaces handling

[neSpecc]

Problem

If block starts with several spaces, they can't be deleted. Caret jumps to the previous block.

Resolves #2429
Resolves #1382
Resolves #1179
Resolves #2521

Cause

There are two whitespace handling in HTML

1. Invisible spaces — all regular trailing spaces, tabs, etc
2. Visible spaces —  

Our Caret.isAtStart method covers only case №1 and treats all whitespaces like "empty".

Solution

I've rewrote isAtStart() and isAtEnd() methods.

1. They were moved from the Caret Module to the "caret.ts" util.
2. Their dependency from this.BlockManager has been removed. Now methods accept input as argument. (so we'll be able to expose them to the api or some package)
3. Their implementation has been simplified. Now we create a range from the block start (or end in case of isAtEndOfInput()) to the caret position, clone its content to the temporary div and check its textContent
4. Now we handle collapsed and visible whitespaced correctly. Util isCollapsedWhitespaces() is used for it.
5. Tests for Backspace/Delete/Left/Right when caret at the start/end added
6. Tests for arrow-navigation through the Delimiter added

[gohabereg]

Looks good overall, just a minor comment

[gohabereg]

👍

[gohabereg]

Similar expression is used several times, maybe worth to extract it to a function. Or allow caret utils to accept undefined

[neSpecc]

I believe that utility methods like isAtStartOfInput() should not handle case when input is undefined . Module should explicitly handle the cases when block has no inputs or there is no currentBlock at all since it is a part of business logic.