Add comma support to /pay command

[werogg]

Information

This PR closes #2355.

Details

Proposed feature:

It doesn't take care of currency-symbol-format-locale but at least it supports both comma and dot as decimal separators.
Just noticed that could be confusing if it is used to perform for example: /pay 1.000 (1k), maybe could think in a different approach making use of the currency locale settings or using it as thousand separator is not expected.

Environments tested:

OS: Windows

Java version: 1.8.0_361

• Most recent Paper version (1.XX.Y, git-Paper-BUILD)
• CraftBukkit/Spigot/Paper 1.12.2
• CraftBukkit 1.8.8

Demonstration:

[pop4959]

This is not an appropriate solution to the issue linked. Any solution here must be able to properly support properly formatted numbers in the locale the user chooses.

For example, 1,000.5 (US) or 1.000,5 (DE) should both evaluate to one thousand and a half.