fix: node+browser compatible ES module import paths

[steabert]

The packages that export ES modules do not have a .js extension on the import paths, which means that the import path does not match an actual file path (since the latter do have a .js extension). This means they cannot be properly resolved by Node.js or the browser.

However, since we're using TypeScript to transpile, and we're using the legacy "node" module resolution, the import paths are preserved and end up not having a .js extension, while the transpiled files to have it.

There are two main ways to deal with this:

1. Use TS configuration {"module":"esnext","moduleResolution":"bundler"} and use a bundler that produces import paths matching an actual file. Note: using TypeScript in this case would leave the imports as-is and require users to use a bundler, so it would not constitute an ES module that just works with node/browser, which is the problem we actually set out to solve.
2. Use TS configuration {"module":"nodenext","moduleResolution":"nodenext"} and use {"type":"module"} in package.json. This will instruct tsc to produce output that matches the package configuration, so ES module in this case.

The advantage with (1) is that it requires no addition of a .js extension to the imports in the .ts(x) source files. Unfortunately this requires TS 5 (we currently use TS 4) and the use of a bundler to build (which we currently do not use). The advantage with (2) is that it requires no changes to how we build, but it does required adding a .js extension to all import paths in the source code.

Since addition of a bundler adds a layer of complexity in maintenance for a repository that should be easily accessible, it seems option (2) is preferable (and also can be done without having to do a major TS ugprade first).

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