ICU-22433 uppercase decomposed greek to decomposed greek and precomposed greek to precomposed greek.

[eggrobin]

Right now Greek uppercasing produces unnormalized text (it always introduces a precomposed dialytika, and always uses the decomposed eta tonos).

This change produces precomposed eta tonos from precomposed eta tonos, as was originally intended, and combining dialytika from combining accents on the preceding vowel; the test checks that NFD is mapped to NFD and NFC to NFC for all test strings.

The change also adds some tests for better coverage of the iota subscript behaviour.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22433
• 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

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