Add peer dependency on eslint

[matwilko]

eslint-plugin-styled-components-a11y is missing a peerDependencies on eslint, as transitively requested by eslint-plugin-jsx-a11y.

While it's not a massive problem, it does make life harder for package managers when trying to optimize package installation, see Implicit Transitive Peer Dependencies.

yarn currently reports as so:

eslint-plugin-styled-components-a11y@npm:2.1.3 doesn't provide eslint, breaking the following requirements:
eslint-plugin-jsx-a11y@npm:6.7.1 → ^3 || ^4 || ^5 || ^6 || ^7 || ^8

[brendanmorrell]

thanks! 2.1.31