Allow '_' to be found in a number token, and ignored for generating the
resulting token. Allow specifying numbers with underscores like
0x1234_5678; 123_534.000_003 ; can include trailing underscores, and just
terminate on the next whitespace or operator...

This is the proposal that was applied for ECMA Script
https://github.com/tc39/proposal-numeric-separator

This is my Proposal
https://gist.github.com/d3x0r/93f8da65f79de66a9736754fdfd99f54

J