fix: es6 export enum

[sapphi-red]

Added es6 export to enum when generating static code from .proto with es6 wrapper.

Example of generated static code

before

$root.Status = (function() {
    const valuesById = {}, values = Object.create(valuesById);
    values[valuesById[0] = "speaking"] = 0;
    values[valuesById[1] = "reviewing"] = 1;
    values[valuesById[2] = "pause"] = 2;
    return values;
})();

after

export const Status = $root.Status = (() => {
    const valuesById = {}, values = Object.create(valuesById);
    values[valuesById[0] = "speaking"] = 0;
    values[valuesById[1] = "reviewing"] = 1;
    values[valuesById[2] = "pause"] = 2;
    return values;
})();