stats & frequency: add a --json flag for JSON output

[rzmk]

Is your feature request related to a problem? Please describe.
Similar to #1718, currently I perform an extra step for getting JSON output from qsv stats and qsv frequency so it may be better for me (both from a refactoring and performance perspective) to not manually perform this step and have qsv handle it for me.

Describe the solution you'd like
A --json flag for qsv stats and qsv frequency with the output formatted like sqlp's --format json flag. No need for pretty-printing the output (could be another flag).