ICU-22261 Add UCONFIG_NO_MF2 flag that can be used to disable MessageFormat 2 functionality

[catamorphism]

This is a followup to #2579 that adds a compile-time flag to disable the MessageFormat 2 functionality.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22261
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
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• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[catamorphism]

I don't think I have the ability to request reviewers, but paging @markusicu @echeran

I'm also not sure if I should file a separate bug for this. (I just used the same bug number as in #2579 for now.)

[richgillam]

Didn't actually test to make sure this works, but it looks good.

Are there MF2 tests in cintltst? If so, they should probably have this guard around them as well.

[catamorphism]

Are there MF2 tests in cintltst?

No, only in intltest (which I assumed was the right place, but I hope someone will tell me if it's not :) )

[richgillam]

Are there MF2 tests in cintltst?

No, only in intltest (which I assumed was the right place, but I hope someone will tell me if it's not :) )

I thought you also had C API, and it'd probably be good to do at least a superficial set of tests to make sure your C wrappers do call through appropriately to the C++ implementation. But I definitely wouldn't ask you to do that now, and since you don't have anything there now, I think you're good to go. :-)

[catamorphism]

I thought you also had C API, and it'd probably be good to do at least a superficial set of tests to make sure your C wrappers do call through appropriately to the C++ implementation. But I definitely wouldn't ask you to do that now, and since you don't have anything there now, I think you're good to go. :-)

Ah, no -- no C API yet.

[markusicu]

lgtm please squash

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

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