[popover2] feat: new props matchTargetWidth, modifiersCustom

[adidahiya]

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

New props for Popover2:

• modifiersCustom allows passing in custom popper.js modifiers to the popper instance
  • note that this name was chosen so that it can appear right next to modifiers in alphabetical sorting.
• matchTargetWidth applies one of our own custom popper.js modifiers to make the popover content match the width of the target (reference) element
  • this behavior was added to Select in [select] feat(Select): new prop "matchTargetWidth" #4841, at which point I suggested it would be nice to add to Popover2 as well, so here we are finally doing it
  • this sets us up to handle matchTargetWidth in a better way in the upcoming Select2 component in [select] feat: new Select2 component uses Popover2 #5306
  • note that we can't use this new prop to replace matchTargetWidth in Select because that component uses Popover, not Popover2

Reviewers should focus on:

Example in docs works correctly

Screenshot

[blueprint-bot]

[popover2] feat: new props matchTargetWidth, modifiersCustom

Previews: documentation | landing | table | demo

[blueprint-bot]

Add unit test

Previews: documentation | landing | table | demo