REPL: fix floating point number parsing

In JS, the expression ".1" is a floating point number.  Issue 4268 concerns the
REPL interpreting floating point numbers that lead with a "." as keywords.  The
original bugfix worked for this specific case but not for the general case:

    var x = [
        .1,
        .2,
        .3
    ];

The attached change and test (`.1+.1` should be `.2`) fix the bug.

Closes #4513.