Unifies the polykey.ts and polykey-agent.ts to make forking to background portable to bundling

[CMCDragonkai]

Description

This unifies the polykey.ts and polykey-agent.ts so that way forking to the background is portable.

Issues Fixed

• Fixes Starting agent in background doesn't work with esbuilt bundle #34

Tasks

• 1. Switching between normal polykey and agent mode with --agent-mode internal flag.
• 2. The globalThis.PK_MAIN_EXECUTABLE is a special property set to always the file path of the main executable. This should also work for pkg, but needs to be tested on the CI with Ubuntu, right now set to __filename, but it can be changed to import.meta.url in ESM.

Final checklist

• Domain specific tests
• Full tests
• Updated inline-comment documentation
• Lint fixed
• Squash and rebased
• Sanity check the final build

[CMCDragonkai]

One of the things I observed is that error reporting from agent start --background isn't good atm, it does not tell you why it failed. Like that there was an error or something. We need to improve the error reporting.

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[CMCDragonkai]

This works for the esbuilt bundle:

[nix-shell:~/Projects/Polykey-CLI]$ ./dist/polykey.js agent start --background
✔ Please enter the password … ******
pid	269435
nodeId	"vj4tcqd7fom79fo3q418g1616739260ngdibpteguvaid49kcakjg"
clientHost	"::1"
clientPort	46311
agentHost	"::"
agentPort	42836

Need to test on the CI for pkg and ubuntu.