ICU-23276 Increase toolutil's STRING_STORE_SIZE

[chrisse74]

Increase STRING_STORAGE_SIZE in toolutil/package.h in order to make adding resources (e.g. additional charsets) possible again.

Checklist

• Required: Issue filed: ICU-23276
• 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