r.stats.quantile: Add JSON support

[NishantBansal2003]

Fixes: #5951

This PR adds JSON support to the r.stats.quantile module. The JSON output looks like:

[
	{
		"category": 27511,
		"percentiles": [
			{
				"percentile": 50,
				"value": 139.62598419189453
			}
		]
	},
	{
		"category": 27513,
		"percentiles": [
			{
				"percentile": 50,
				"value": 143.7049102783203
			}
		]
	},
	{
		"category": 27518,
		"percentiles": [
			{
				"percentile": 50,
				"value": 122.53437805175781
			}
		]
	}
]

This PR includes the following changes:

1. Adds a format option with plain, csv, and json modes for output formatting.
2. Adds format = csv option; the -t flag is now deprecated.
3. If no separator is specified, the default is :; If the format is CSV, the separator is a comma.
4. Adds tests covering each of the new formats.
5. Adds a Python example to the documentation for parsing JSON output.

[petrasovaa]

Works great! Thank you!