ICU-22939 Declare ubidi_isolate() with U_CAPI linkage

[catamorphism]

Declare ubidi_isolate() with U_CAPI linkage for consistency with other functions. This entails changing its return type from UnicodeString to UnicodeString* so that it can have C linkage.

Checklist

• Required: Issue filed: ICU-22939
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN 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
• Approver: Feel free to merge on my behalf

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