MultiMode Lexer defaultMode should not be implicitly defined.

[bd82]

Currently a MultiMode lexer uses the "first" mode in the definition as the default mode.
So in this example the defaultMode will be "numbers_mode"

// old deprecated format
var multiModeLexerDefinition = {
    // the default (initial) mode is "numbers_mode" it is implicitly defined as the first key in the definition object.
    "numbers_mode": [One, Two, Three, ExitNumbers, EnterLetters, Whitespace],
    "Letter_mode":  [Alpha, Beta, Gamma, ExitLetter, EnterSigns, Whitespace],
    "signs_mode":   [Hash, Caret, Amp, ExitSigns, EnterNumbers, Whitespace]
};

The problem is that the default mode relies on the order of iteration on javascript objects
which is left undefined in the ECMAScript specifications and is thus implementation dependent.

A more robust way would be to define the defaultMode explicitly:

// new format
var multiModeLexerDefinition = {

    modes : {
        "numbers_mode": [One, Two, Three, ExitNumbers, EnterLetters, Whitespace],
        "Letter_mode":  [Alpha, Beta, Gamma, ExitLetter, EnterSigns, Whitespace],
        "signs_mode":   [Hash, Caret, Amp, ExitSigns, EnterNumbers, Whitespace]
    },

    // the default mode is explicitly defined.
    defaultMode : "numbers_mode"
};

[bd82]

Not that this is a breaking change which requires slight modification to existing MultiMode Lexers.