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