Fix no-multiple-empty-lines rule

[vladshcherbin]

From readme:

19.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint: no-multiple-empty-lines

Which has a bad example:

// bad - multiple empty lines
var x = 1;


var y = 2;

Currently, max empty lines is set to 2 and above example passes as a correct one. Same goes for other useless empty lines - between function declarations, etc.

[vladshcherbin]

Since it's kinda breaking change it would be great to ship this with next major version, with eslint v7 support I believe.

[ljharb]

This would be considered semver-minor, since the guide already specifies "no multiple empty lines".

I can't recall why this was set to 2 originally - git suggests it was not intentional, and just matches the default.