Support retrieving quickpick items when using a multi-select quickpick

[ewanharris]

This is one attempt at a new API to solve #690, rather than using getQuickPicks to interact with a multi-select quickpick it introduces a new API getCheckboxes.

An alternative solution would be to make getQuickPicks take an argument specifying the quickpick as a multi-select such as getQuickPicks({ canSelectMany: true })

Before submitting your PR, please review the following checklist:

• CONSIDER adding a new test if your PR resolves an issue.
• DO keep pull requests small so they can be easily reviewed.
• DO make sure tests pass.
• DO make sure any public APIs changes are documented.
• DO make sure not to introduce any compiler warnings.

Before merging the PR:

• CHECK continous integration of main branch is green.
• CHECK pull request check job is green.
• CHECK all pull request questions/requests are resolved.
• WAIT till PR is approved by at least 1 committer.

[djelinek]

I am not able to handle this PR as part of upcoming release, but I will do for next release. Marking this PR as prio for next PR review iteration.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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