fix(deps): update dependency y-prosemirror to ^1.2.15 (stable30)

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
y-prosemirror	^1.2.12 -> ^1.2.15

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Release Notes

yjs/y-prosemirror (y-prosemirror)

v1.2.15

Compare Source

• wrong property name for #​173 fix eb4bf4c

v1.2.14

Compare Source

• possible fix for #​173 - cursor selection exception ee5af2e

v1.2.13

Compare Source

• Merge pull request #​171 from gvergnaud/fix-runtime-error-when-adding-y-sync-plugin 0eef55a
• fix: avoid throwing an exception if the current selection no longer makes sense when initializing ySyncPlugin a501e0b

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