Implement 'organize imports' functionality

[rvanasa]

Sorts and groups Motoko imports on pressing the "Organize imports" key binding.

Here is an overview of the current grouping and sorting rules:

• Sort alphabetically by path, import name, and then field name
• Move named module imports above explicit field imports for the same path
• Group by prefix (ic:, canister:, mo:, and then relative paths)

TODO:

• Automatically remove unused imports

Progress towards #123.