Add .cljr as default Clojure file extension

[PEZ]

• Fixes .cljr files not treated as Clojure by default #2194

My Calva PR Checklist

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• 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.
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• 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.
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  • Tested the particular change
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Ping @PEZ, @bpringe, @corasaurus-hex, @Cyrik