fix: clear nodes when cursor at start of empty isolating parent

[bdbch]

Please describe your changes

This fix will solve the issue that when a cursor is inside an isolating node and the isolating node is empty, the nodes inside are not cleared similar to how they are cleared on the document.

How did you accomplish your changes

Inside the keymap.ts we also check for the current textNodes parent (or direct parent if it is not a textNode) and if said parent is isolating. If we are inside an isolating node, we use the relative parentOffset to check that we're at the start of a node.

How have you tested your changes

• Used an isolating node to test the behavior, nodes were cleared correctly
• Used it directly in the document: nodes were still cleared correctly

How can we verify your changes

1. Create an isolating node
2. Add content
3. Make this content a headline
4. Use backspace to completely remove the content
5. Use backspace again to clear the headline and turn it back into a paragraph

Checklist

• The changes are not breaking the editor
• Added tests where possible
• Followed the guidelines
• Fixed linting issues

Related issues

No related issues

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