chore: migrate src/helper to typescript (#5689)

This PR migrates the helper module to TypeScript. It's a bit of a bigger
change than others because I decided to move away from the helper class
with static methods and move towards a simpler set up where the module
is a bunch of functions. I still expose them under the `helper`
namespace to avoid this being a big change - we can update that later
when we migrate to ESM.

We do have to do some unfortunate wrangling of the promisify function.
Ideally I'd like to rely on the Node one (and the type defs) but that
doesn't work in Browserify land. I've stuck with the promisify in
`helper.ts` but pulled it into its own module to enable me to leave a
comment clarifying why we use it and the context. We can solve this with
a better web bundling story but that work is lower priority right now
than getting the `src/` directory into TypeScript.