Migrate to TypeScript and use Map

[jackfranklin]

This PR:

• migrates the internal events object to be a Map, rather than an object.
• migrates src/index.js to TypeScript
• updates the bundling to use Microbundle and drops all the custom rollup builds
• removes any reference to Flow types
• updates the TS types to allow event types to be a string or symbol (now we're using a Map they could theoretically be anything).
• adds ESLint TypeScript rules

Important to note this is very much a breaking change for multiple reasons!

@developit I'm happy to update the CHANGELOG once this is good to ship.

[developit]

It's within 3 bytes! :D

[developit]

Published as 2.0.0 (prerelease) - install it via npm install mitt@next.