feat: add type declarations for @analytics/segment

[VitorLuizC]

I've written the type declarations file for @analytics/segment plugin and included analytics as peerDependencies, because its AnalyticsPlugin type is used. I've also removed README.md from the "files" array in package.json because it's always included by npm, like the LICENSE file.

[VitorLuizC]

@DavidWells if you prefer I can move these type declarations to @types (https://github.com/DefinitelyTyped/DefinitelyTyped) instead.