Allow plugins to validate their options

[edsrzf]

This introduces a new validate method to the Plugin interface so that plugins can validate their options. It also implements validate for all plugins that have options. It also adds a PluginOptionsError type to ts-migrate-server which should be thrown when validation fails.

The motivation was originally described in #74, where I proposed a configuration file format. One consequence to a configuration format is that plugins will need to be more defensive about validating their options.

Internally the validation is implemented using JSON schema, with additional JS validation where JSON schema is not expressive enough (only the member-accessibility plugin, currently).

[Rudeg]

Thanks for working on this!

The interface additions, including details on names and such:

• The validate method and signature.
• The PluginOptionsError class.

Lgtm, small nit comment about default error message.

Any thoughts on JSON schema or the json-schema package in particular

Usage of json-schema makes sense to me.

Would it make more sense to go all-in on JSON schema and expose an optionsSchema instead of the validate method? (I noticed that @typescript-eslint does this in its plugins. For example) Consider though JSON schema by itself is not expressive enough to validate the regex strings that member-accessibility accepts. Or maybe I've gone overboard with the regex validation.

With current state of the options - validate method is fine imo.

[Rudeg]

@edsrzf sorry for the delay, lgtm!