Move Vue plugin into separate file

[bakerkretzmar]

This PR moves the Vue plugin added in #407 into it's own file and builds it separately from the main package. It adds new microbundle scripts to build these files because microbundle doesn't yet support multiple entry and output files.

This does basically duplicate the whole package, since all of Ziggy's core is included in the new bundle too, but that shouldn't matter since users of the Vue plugin will only import that module, not the main route one.

Using the Vue plugin would now look like this:

import { ZiggyVue } from 'ziggy-js/dist/vue';

Closes #422, but in v2.0 we will probably stop using default exports in favour of all exports being named, which will allow this:

import { ZiggyVue } from 'ziggy-js';