Linter: Fixes for ESLint 7.x

[wcjohnson]

Fix the linter to test against ESLint 7.x. This mostly has to do with the usage of the ESLint API and the test suite.

[wcjohnson]

The main obstacle here is the plugin's implementation of reaching inside the linter's module and monkey patching broken rules.

New approach should be that the default config exported by the linter module simply disables broken rules, we re-implement the new rules as @lightscript rules, and then enable those replacement rules. This will also need to be documented so users of the linter know which rules correspond with which.