[Shortcuts]: Prototype UI for configuring shortcuts

[fallaciousreasoning]

We should add a prototype behind the brave://flags#brave-commands flag for customizing shortcuts. In the prototype, it should be possible to:

1. Add shortcuts (to commands with existing shortcuts, or to other Chromium commands, such as pin & duplicate tab).
2. Remove existing shortcuts (if you don't like Ctrl+P to print, should can remove it).
3. Reset shortcuts to whatever they are by default

This is a follow up to #28329 and a prerequisite to #354