Add Code Entry functionality

[Pawkkie]

Description

Adds code entry functionality. This is the functional skeleton to support event scripting where the player enters a string that gets checked against a specified list, and actions are taken depending on what the string matches.

This implementation does not include any event script changes, but it does include a thorough guide on how to use the two new specials EnterCode and GetCodeFeedback in event scripting. All functionality for the codes themselves is also left to the user, and a tutorial for how to add your own is included in the aforementioned guide.

In very abbreviated, you can use the following in an event script:

special EnterCode
waitstate
special GetCodeFeedback

To prompt the player with a text box for entry and compare that entry against results. GetCodeFeedback is flexible such that you can perform operations using C code for particular code entry in that function, or use the results it returns to lead to specific event scripting, for things that may be better handled in scripts (ie. givemon). The tutorial document covers examples of both of these cases.

People who collaborated with me in this PR

@PCG06 for the Mystery Gift iteration

Discord contact info

@Pawkkie