feat: support custom inline attributes

[mrverdant13]

Details

Expose a way to register custom inline attributes.

Devs often need a way to use custom attributes depending on some specific use cases. i.e. mentions, tags, etc.

Related issues and discussions

• Mentions: Is it possible to have a custom attribute and styling for a given set of text? #564
• Custom image: [Mobile] How to create custom attributes for image #559
• Custom embed parameters: Discussion: Delta model #569

I am working on a really simple PR that might enable this feature but would like to have additional feedback once I submit it.

[singerdmx]

No I am against the idea

[singerdmx]

Anything needs to be added should be directly added into repo

[singerdmx]

Such PR will not be approved

[singerdmx]

Custom styling is already supported
No custom attributes
If you need any new attributes, it should be in the repo

[mrverdant13]

That's a shame.
This would promote the development of external packages to be plugged to flutter_quill.
No worries.

[singerdmx]

We have enough bugs to fix, e.g. #561 (I think you are working on this).
We don't need unnecessary complication to the repo. This will only make maintainability worse.
Also we don't want people just to use this repo without contributing to it such as adding more inline attribute feature.