chore: add esbuild bundler

[mR4smussen]

eventually this closes #70

This PR introduces esbuild as a project bundler. Esbuild is built on Go, allowing it to get extremely good compile times.
Also for the Flix extension to be used in a vscode web environments like github.dev and vscode.dev we will need to use a bundler, since vscode can only load a single file for an extension when running in a browser.

I wasn't quite sure of the best way to test this PR. When building the project, I get no errors (and I get the bundled client/out/extension.js file), and when launching the extension, it works like usual.

[magnus-madsen]

Does this require one to have Go installed or does it work out of the box?

[mR4smussen]

It works out of the box

[magnus-madsen]

Ill take a look later today :)

Today you have time for a meeting next week?

[mR4smussen]

you have time for a meeting next week?

Sure, let’s coordinate over Gitter

[magnus-madsen]

It seems to work, but were not there yet, because I still see:

This extension consists of 3178 files, out of which 2824 are JavaScript files. For performance reasons, you should bundle your extension: https://aka.ms/vscode-bundle-extension . You should also exclude unnecessary files by adding them to your .vscodeignore: https://aka.ms/vscode-vscodeignore
 DONE  Packaged: X:\Work\GitHub\flix\vscode-flix\flix-0.88.0.vsix (3178 files, 3.26MB)