RFC: Improve the UX for aliases

[cristianl]

This is a recurring feature for some libraries.

I think users need to configure it more often (and are just silently ignoring the resolver misses?) than lookup paths (usually tied to ecosystem defaults, node_modules for js/ts etc) and setting them in the user settings introduces unintended behavior for other projects.

I'm thinking of adding a command to set a alias: dir mapping for the current scope.

Considerations:

• Rarely-used commands pollute the command palette

Cons:

• Need to handle accidental hits gracefully

Pros:

• Less risk of setting scope keys incorrectly
• Not needing to find the appropriate scopes (#63 (comment))