✨ Allow automatic conversion of units to metric

[TheOnlyTails]

Describe the solution you'd like
When passing the --metric option to the CLI, it should attempt to convert as much data provided in imperial units to metric units:

• inches to centimeters
• feet to meters (the standard for D&D is that 5ft/1 square is 1.5m)
• miles to kilometers
• pounds to kilograms
• ounces to grams
• gallons to liters
• etc...

This would be very helpful for those of us who use the metric systems and can't bear seeing imperial units used everywhere.

Additional context
I got the idea from the Metric Beyond browser extension, which does this for D&D beyond sources and character sheets.