Add :append option to -mainlog in prism CLI

[davexparker]

Summary

• Extracts -mainlog argument processing into processMainLogSwitch() in PrismCL.java
• Adds support for a :append suffix (e.g., -mainlog out.log:append) to open the log file in append mode rather than truncating
• Removes the now-redundant mainLogFilename field (was effectively a local variable)

Test plan

• ./bin/prism <model> <props> -mainlog out.log — creates/truncates out.log as before
• ./bin/prism <model> <props> -mainlog out.log:append — appends to out.log on repeated runs
• -mainlog out.log:unknown — exits with "Unknown option" error message
• -mainlog with no argument — exits with "No file specified" error

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