fix (mention/heading): clashing heading markdown command and mention/suggestion with #

[VV-EE]

Please describe your changes

Fixes #2570
Now mention nodes with # will preempt the heading markdown input rule

How did you accomplish your changes

Instead of using addInputRules I've added the heading input rule as a separate ProseMirror plugin. After this higher priority extensions will be ahead of this input rule. For the same reason I set the Mention node's priority to 101.

How have you tested your changes

Added 3 E2E tests, one checks if there's no heading created is something is selected from the mention lis, one checks that we can still use the heading shortcut if we don't select anything, one checks if two mention plugins with '@' and '#' can work together.

How can we verify your changes

Run the E2E test, or visit preview/Examples/HeadingMultipleMention

Remarks

Added some documentation to Mention about higher priority.

Checklist

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

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[svenadlung]

Pretty cool! Thank you so much 🙌

[svenadlung]

Sorry to be so nit-picky, but can you please insert breaks before and after?

[VV-EE]

I've put the comment on a separate line, I hope that's what you meant.

[svenadlung]

@bdbch what do you think? What do you think of this solution?

[svenadlung]

@VV-EE we are still in internal discussion to find a better way to respect the priority of the input rules in the ExtensionManager.

[VV-EE]

I get it, this is a workaround, not a solution to the underlying problem. I thought about adding an extra forcePriority prop to ExtensionManager and order all plugins with that too, but I don't know how often this problem happens. If it's a one-off then a workaround is better IMO.