ICU-22942 MF2 ICU4C: NFC-normalize names and keys according to spec

[catamorphism]

This PR adds NFC normalization of variable names and .match keys, consistent with the spec change in unicode-org/message-format-wg#885.

The tests added by this PR are also in a PR against the spec repo, unicode-org/message-format-wg#904 , and can be removed once that PR lands and the ICU tests are updated from the spec repo.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22942
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[jira-pull-request-webhook]

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[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

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[srl295]

Good, and good for getting the guards right.

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

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