Allow avoid merging layers with pen tool

# Reproduction
When drawing with the pen tool, a user may wish to keep two layers separate. This could be useful for e.g. applying different fills or animations. However if the anchor shares a position with an anchor from another layer, it will always automatically merge.

https://github.com/user-attachments/assets/78a04e27-e7d0-4303-8882-aa034ef2bfe9

# Fix
- Indication when placing the anchor will automatically merge layers (e.g. in the tooltip). It is currently very hard to tell.
- Option to avoid merging the layers even when they have the same anchor position.

# Code
https://github.com/GraphiteEditor/Graphite/blob/cc7d700bfda6961fece0b6125289ef39c9caa0a5/editor/src/messages/tool/tool_messages/pen_tool.rs#L1883-L1900

Start in editor/src/messages/tool/tool_messages/pen_tool.rs around lines 1883-1900 and trace the anchor-placement path used by the pen tool. Determine how the merge is triggered and where tooltip feedback is defined. Done means users can tell when placement will merge layers and can choose to keep layers separate at the same anchor position.

Scritto dal modello di indicizzazione a partire dal testo della issue.