Improve TypeScript usage

[shalvah]

(Apologies for the size of the PR; the changes involved turned out to be a little more than anticipated.)

This PR:

1. Adds @types/express to our dependencies, fixing Typescript module error #604
2. Adds some missing null checks. A number of places had a check for if (typeof thing === 'object'), with the subtle bug that null in JS also returns object.
3. Improves typing. A number of types (some of which were user-facing) were incorrect or incomplete. I've fixed them so they're (mostly) correct, providing better IDE suggestions.
4. Switches to (mostly) automated generation of type declarations. This is done by enabling declarations in our tsconfig.json, then updating our Rollup config to point to the right paths for each build. However, this generates separate type decl files for the two builds, so there's a manual honeybadger.d.ts file that re-exports them as one file as before. Additionally, users can still import the browser/honeybadger.d.ts or server/honeybadger.d.ts files like before (with the added advantage of accurate types for the build).
5. Updated the README to explain this for first-time contributors.

[shalvah]

Also: while working on this, I noticed there's nothing in the docs about working with TypeScript or ES Modules. For instance, should users do import Honeybadger from '...' or import * as Honeybadger from '...'. It's something that would be worth addressing eventually.

Not a priority (React doesn't even support native ESM yet) but I think some folks will eventually start asking about it as ESM gains traction.

[joshuap]

This is beautiful. Thanks!

[joshuap]

@shipjs prepare

[github-actions]

@joshuap shipjs prepare fail

[shalvah]

👍