Add autocompletions for functions and variables

[doonv]

Objective

Fixes #26

Solution

There's a new command (UpdateAutoComplete) that the UI calls when the command is changed or when the cursor moves (currently does it every frame). Then the UpdateAutoComplete command calls CommandParser::completion which then makes the current command parser return a list of CompletionSuggestions, which is then displayed to user.

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Changelog

• Added autocompletions
  • The CommandParser trait has a new optional method: completion, every time the UI calls this method every time the command changes or the cursor changes.
  • The Builtin-parser now implements the new optional CommandParser method.
  • Added CompletionSuggestion, which represents a completion suggestion.
  • ...other stuff i probably forgot about