ICU-22087 Export a non-recursive canonical decomposition supplement for ICU4X

[hsivonen]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22087
• 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
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

Tests are in unicode-org/icu4x#2099

[sffc]

Suggestion (optional):

1. Change this function to take nonRecursive32 and nonRecursiveBuilder as arguments, or move it to a new function called computeDecompositionsInner
2. Create a new function computeDecompositionsAndWriteNonRecursive and move the file-writing machinery to it
3. At the call site, change computeDecompositions("nfd", ...) to computeDecompositionsAndWriteNonRecursive("nfd", ...)