Set up build step for extension

[ComputerBread]

could be done at the same time as #12, but I still want to make an issue about it:

https://bun.sh/docs/bundler#define

• [ ] use define to set a global identifier storing the backend url
• [ ] make a build step for
  • [ ] building the chrome extension (dev)
  • [ ] building the chrome extension (prod)
  • [ ] building the firefox extension (dev)
  • [ ] building the firefox extension (prod)
• [ ] start looking at github action: https://bun.sh/guides/runtime/cicd