Added MessageContext.hasMessage and MessageContext.getMessage methods.

    Using the MessageContext.messages map for getting raw messages is
    deprecated now.  Instead, use the two dedicated methods: hasMessage
    and getMessage.

    Before:

        const msg = ctx.messages.get(id);
        const txt = ctx.format(msg);

    Now:

        const msg = ctx.getMessage(id);
        const txt = ctx.format(msg);

The compat build is now transpiled using rollup-plugin-babel.

    This ensures that the "use strict" pragma is scoped to the UMD
    wrapper.  It also correctly parses the top-level "this" keyword
    (which the previous setup turned into "undefined").