Introduce the FluentParser and the FluentSerializer classes.

    Instances can be used to store config for multiple parse/serialize
    operations.

    The fluent-syntax package still exports the `parse` and `serialize`
    helpers.  For more fine-grained control, `FluentParser` and
    `FluentSerializer` may be used for instantiation.

Build compat.js using babel-plugin-transform-builtin-extend.

    This ensures the correct behavior of `err instanceof ParseError`.

The compat build is now transpiled using rollup-plugin-babel and
babel-plugin-transform-builtin-extend.

    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").