fix: specify globals as a required peer dependency

[G-Rath]

Because this plugin imports globals directly at runtime it should specify it as a dependency - technically, it could be a standard dependency, but I've gone with specifying it as a peer dependency to reduce maintenance burden and improve package deduplication, since

1. it's core to ESLint configs, including being a dependency of ESLint itself so it's all but guaranteed to be present (which is why this hadn't come up sooner)
2. it does breaking changes more frequently than other packages, because it's a breaking change to change globals on groups, but the groups themselves (especially the browser group that this plugin uses) are very stable

I have set v13.21.1 as the min. version allowed because that is a bugfix to address a rogue space in a property name that makes ESLint error, but technically most if not all versions of globals are compatible since the browser group has been around for quite a while

Resolves #1269