* chore: migrate src/ExecutionContext to TypeScript
I spent a while trying to decide on the best course of action for
typing the `evaluate` function.
Ideally I wanted to use generics so that as a user you could type
something like:
```
handle.evaluate<HTMLElement, number, boolean>((node, x) => true, 5)
```
And have TypeScript know the arguments of `node` and `x` based on those
generics. But I hit two problems with that:
* you have to have n overloads of `evaluate` to cope for as many number
of arguments as you can be bothered too (e.g. we'd need an overload for
1 arg, 2 args, 3 args, etc)
* I decided it's actually confusing because you don't know as a user
what those generics actually map to.
So in the end I went with one generic which is the return type of the
function:
```
handle.evaluate<boolean>((node, x) => true, 5)
```
And `node` and `x` get typed as `any` which means you can tell TS
yourself:
```
handle.evaluate<boolean>((node: HTMLElement, x: number) => true, 5)
```
I'd like to find a way to force that the arguments after the function do
match the arguments you've given (in the above example, TS would moan if
I swapped that `5` for `"foo"`), but I tried a few things and to be
honest the complexity of the types wasn't worth it, I don't think.
I'm very open to tweaking these but I'd rather ship this and tweak going
forwards rather than spend hours now tweaking. Once we ship these
typedefs and get feedback from the community I'm sure we can improve
them.
* chore: migrate src/JSHandle to TS
There's a few TODOs in here that all depend on typing the
`ExecutionContext.evaluateHandle` properly so that you can properly
declare what types you're expecting back. Once I've done that file (it's
next on my list) I will loop back and improve the types here, fixing
these TODOs.
* Fix doclint for {}
This updates our `tsconfig.json` so it emits our JavaScript files as
well as type checking them. We compile into `./lib` which we then ship
in our npm package. The source code has moved from `./lib` into `./src`.
Because the `src/` directory is exclusively JS files, this change is a
no-op in terms of code functionality but is the first step towards being
able to replace `src/X.js` with `src/X.ts` in a way that allows us to
migrate incrementally.
The `lib` directory is gitignored, and the `src` directory is
npmignored. On `npm publish` we will now run `npm run tsc` in order to
generate the outputted code.
