Clarinet extension handles

[lgalabru]

Add aliases to clarinet run commands.
For instance,

clarinet run generate-stacksjs-helper

Would run the latest version of that script. That would be more convenient than

clarinet run https://deno.land/x/[email protected]/ext/stacksjs-helper-generator.ts

• we'd be able to leverage shell-completions, help, etc.

[hugocaillard]

Closing this issue because clarinet run will soon be deprecated #1065