feat: allow to override link validation check, and accept mailto and other links by default

[EchoEllet]

Description

Allow the toolbar to insert links like mailto by default, allow to override the link validation with a callback rather than Regex to give users more control, improve doc comment of customLinkPrefixes and legacy code and confusing naming, adharing to common best practices.

• The customLinkPrefixes which is in the editor, is only used to check whether this link is valid, if it's invalid, https:// will be appended as prefix to the link (google.com -> https://google.com).
• The validateLink in QuillToolbarLinkStyleButtonOptions allows the validation check to be overridden to determine whether this link can be inserted.

However, AutoFormatMultipleLinksRule still uses the old regex and doesn't support mailto and the others.

To test this branch:

dependency_overrides:
  flutter_quill:
    git:
      url: https://github.com/singerdmx/flutter-quill.git
      ref: fix/1776

Related Issues

• Partial fix customLinkPrefixes isn't plumbed throughout library #1776
• Fix Insert URL tool only accepts links starting with http:// or https:// #2492
• Related customLinkPrefixes parameter - makes possible to open links with custom protocol (deeplinks) #1164
• Related Pasting paragraph with links doesn't linkify #1432

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Refactor: Code reorganization, no behavior change.
• ❌ Breaking: Alters existing functionality and requires updates.
• 🧪 Tests: New or modified tests
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Build/configuration changes.

[CatHood0]

LGTM. I'll test it as soon as possible.

[CatHood0]

My bad jasj.

[EchoEllet]

I will change to draft and add the tests soon before merging this PR.

[EchoEllet]

This import should be updated to use the insert.dart file directly.