Change the naming convention for $id to match file names

[pjohnmeyer]

Some schema loaders support loading schemas from a "prefix":

https://opis.io/json-schema/2.x/php-loader.html#registerprefix-method

Currently, our IDs and our filenames don't match, e.g.:

file: .../schema/enums/CompensationType.schema.json
$id: https://opencaptablecoalition.com/schema/enums/compensation_type

If we make the $id values match the file path, in this case https://opencaptablecoalition.com/schema/enums/CompensationType.schema.json, it will enable simpler schema loading.

[JSv4]

I'm in. Sounds like a great idea.

[JSv4]

Making this change via issue #120