Adding `sideEffects: false` to package.json

[neoncube2]

Hello! :)

Would it make sense to add sideEffects: false to package.json? This can enable more tree-shaking, in some cases: documentation

Other popular libraries, such as axios, have been adding this flag: axios/axios#5025

I'd be happy to open a PR :)

[mcschwa]

go ahead 👌

[neoncube2]

@mcschwa Done! :)

PR #212

[neoncube2]

@mcschwa BTW, it looks like I'm not using this package in my project, so I wasn't able to test these changes. It'd probably be smart to test these changes with a project that directly uses mitt