[Feature]: Theme dev - Browser selection

[kmnunley]

What area(s) will this request affect?

Theme

What type of change do you want to see?

New feature

Overview

Add the ability for users of the CLI to select which browser to use when opening theme previews / editors with the following priority:

1. Environment / command flag (e.g. '--browser chrome' or 'browser = "chrome"' in shopify.theme.toml)
2. VSCode preferences (workbench.externalBrowser workspace setting or global setting)
3. System default browser

Motivation

Cross-browser testing is currently cumbersome. The CLI only supports opening with the system default browser. This change would allow multiple environments to be created to quickly switch between browsers to expedite testing.

Additionally, this would assist users who develop on a different browser than their system default.
For example:
If a user chooses a third-party chromium based browser or Firefox as their daily-driver, they may still need or want to develop using vanilla Chrome.