Multicursor - Refactors - EditableDocument uses selections

[riotrah]

What has changed?

Addresses #610. Another step in refactoring all cursor/selection related code to assume multiple selections without affecting user functionality. Dependent on #2413.

• Altered the .selection property of EditableDocument to be an array, .selections instead
• Updated all call sites, including tests

My Calva PR Checklist

I have:

• Read How to Contribute.
• Directed this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I have changed the PR base branch, so that it is not published. (Sorry for the nagging.)
• Made sure there is an issue registered with a clear problem statement that this PR addresses, (created the issue if it was not present).
  • Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.
  • Referenced the issue I am fixing/addressing in a commit message for the pull request.
    • If I am fixing the issue, I have used GitHub's fixes/closes syntax
    • If I am fixing just part of the issue, I have just referenced it w/o any of the "fixes” keywords.
• Figured if anything about the fix warrants tests on Mac/Linux/Windows/Remote/Whatever, and either tested it there if so, or mentioned it in the PR.
• Added to or updated docs in this branch, if appropriate
• Tests
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
• Formatted all JavaScript and TypeScript code that was changed. (use the prettier extension or run npm run prettier-format)
• Confirmed that there are no linter warnings or errors (use the eslint extension, run npm run eslint before creating your PR, or run npm run eslint-watch to eslint as you go).

Ping @PEZ, @bpringe, @corasaurus-hex, @Cyrik

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[PEZ]

You checked two boxes, and nonetheless. 😄

I think there is an open issue about multicursor paredit that you can refer this PR too. We usually use language as “Adderessing #XXXX” when we are not closing or fixing issue with some commits.