Add browser option

[00-kat]

Originally from anyrun-org/anyrun#173.

This allows you to choose a custom browser.

Examples:

• You have both Firefox and Chrome. Firefox is your default but you want websearch to use Chrome:

  Config(
    browser: "chrome",
  )

• You want to pass custom flags to your browser:

  Config(
    browser: "firefox --new-window",
  )