Report power as JSON

[nataliakokoromyti]

Adds JSON output format for power analysis reports with per-cell and summary power breakdowns (internal, switching, and leakage components).

Features:

• Added -format flag to report_power command
• Supports "text" (default) and "json" output formats
• Per-cell power breakdown (internal, switching, leakage)
• Summary power statistics
• Includes tests

Based on Silimate/OpenSTA implementation with upstream integration.

[jjcherry56]

There is no point in mucking up the non-json reporting by filling it with "if" statements because there is virtually no code sharing.