ICU-23017 Add long conversion functionality to ICU4C

[younies]

Description:

• Introduced new source files: long-conversion-string-to-long.cpp and long-conversion-string-to-long.h for converting strings to long integers.
• Updated project files (i18n.vcxproj, i18n_uwp.vcxproj, and their filters) to include the new files.
• Modified sources.txt and dependencies.txt to reflect the addition of the long conversion functionality.

This enhancement supports better handling of long integer conversions in the ICU library.

Checklist

• Required: Issue filed: ICU-23017
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 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