Enable support for eslint v7

[chawes13]

Resolves #24

Installing eslint@^8.0 as a dependency was causing some issues when trying to use this configuration on projects with eslint@^7.0. This makes it impossible to update the configuration on older projects without also requiring an update to eslint@^8.

There is still a breaking change, which is the requirement to use babel.config.js instead of .babelrc for the parser to work correctly. I'll update the release notes with this information accordingly.

[mwislek]

Not sure I totally follow "eslint": "^8.0.0" as a "devDependency" and "eslint": "^7.5.0 || ^8.0.0" as a "peerDependency". I think the "peer" makes sense: the including dependent is expected to have a version of eslint installed at either 7.5 or 8 (is there a reason this isn't >=7.5?) I don't understand the "devDependency" though.

Also, why is @babel/core a "peerDependency" when this module only seems to use @babel/core for its own "dev" purposes.

[chawes13]

Not sure I totally follow "eslint": "^8.0.0" as a "devDependency" and "eslint": "^7.5.0 || ^8.0.0" as a "peerDependency". I think the "peer" makes sense: the including dependent is expected to have a version of eslint installed at either 7.5 or 8 (is there a reason this isn't >=7.5?) I don't understand the "devDependency" though.

eslint is required as a dev dependency because we're using eslint in our tests!

Also, why is @babel/core a "peerDependency" when this module only seems to use @babel/core for its own "dev" purposes.

@babel/core is a peer dependency of @babel/eslint-parser. Since this package isn't requiring them as dependencies, we're passing that requirement on to the consumers of this package. Maybe we don't need to do that since it'll get picked up when installing @babel/eslint-parser? It felt more communicative to me at the time, but doesn't make much sense now! Thanks for flagging that.