Support the `default` JSON Schema keyword

Would be nice to be able to specify default values for the rule options in the schema.

See: https://github.com/epoberezkin/ajv#assigning-defaults

I think all you have to do is to set the useDefaults: true option in https://github.com/eslint/eslint/blob/2af94466fe8f87058fad4bd168958d2a7612a79d/lib/util/ajv.js#L18-L23

[platinumazure]

This issue was accepted in the 2018/09/13 TSC meeting.

(Edit: Also, I noted that ajv’s validate method does mutate the argument passed in once useDefaults: true is enabled. So we’ll want to make sure we’re defensively cloning user options or otherwise avoiding problems as best we can)

[platinumazure]

TSC Summary:

JSON Schema supports a “default” keyword which allows schema authors to specify default values for optional properties in an object schema (among other cases). By allowing schemas to have defaults (a 1-line change in the ajv wrapper code), both core rules and custom rules could have default values in schemas.

Additionally (although maybe as a separate proposal), we could write some generic code when creating rule instances which would hydrate default values for any rule options not provided by the user, which would allow us to remove some code in all rules related to option defaults. There shouldn’t be any impact to users because the default keyword only has its semantic meaning as a keyword outside of property lists (i.e., rules with a “default” named option should not be affected).

TSC Question: Should we enable use of “default” keyword in rule schemas?