This commit adds a toggle functionality to the `brush.asset_activate`
operator that makes it behave similarly to the `paint.brush_select`
parameter of the same name.
When the operator has this option enabled, using the operator or
pressing the relevant key will either:
* Activate the specified brush and store it if the current brush does
not match the specified brush
* Activate the previously stored brush if it exists.
This option is exposed in the keymaps and enabled by default for the
Sculpt Mask brush.
This allows, for example, users to press 'M' to switch to the mask brush
and then press 'M' again to switch back to their previously active
brush.
Partially addresses this RCS submission: [1]
[1] https://blender.community/c/rightclickselect/1VwZ/
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### Notes
This commit does not currently clear the `AssetWeakReference` when switching paint modes.
Pull Request: https://projects.blender.org/blender/blender/pulls/138845
9e1e9b0859