Add a mod input controls API

[psyGamer]

Provide a way for mods to create controls (VirtualButtons, etc.) and automatically add them to the controller config.
They should probably be namespaced under mod ID to prevent conflicts.

I'm not sure how they would be defined. Maybe part as mod settings (which would need to be implemented first) or completely separate like in Vanilla.

When going with option 2, it could look something like this (inspired by Everest):

Module Class:

public override Type ControlsType => typeof(MyModControls);
public static MyModControls Controls => (MyModControls) Instance._Settings;

MyModControls.cs:

[ModControl(Default = [new(Keys.F7)])] // NOTE: I'm not so sure about the "ModControl" name..
public static readonly VirtualButton SaveState = new("SaveState");

[axiand]

Setting this as a goal for 0.6 to go along with #52

[jasminegamedev]

This has been completed as of #93
New Virtual buttons and virtual sticks can be added by adding something like this to the Mod settings file:

[DefaultBinding(Keys.LeftControl)]
[DefaultBinding(Axes.LeftTrigger, 0.4f, false)]
[SettingName("Crouch")]
public VirtualButton CrouchButton { get; set; } = new VirtualButton("Crouch");

[DefaultStickBinding(StickDirection.Up, Keys.W)]
[DefaultStickBinding(StickDirection.Up, Axes.RightY, 0.0f, true)]
[DefaultStickBinding(StickDirection.Down, Keys.S)]
[DefaultStickBinding(StickDirection.Down, Axes.RightY, 0.0f, false)]
[DefaultStickBinding(StickDirection.Left, Keys.S)]
[DefaultStickBinding(StickDirection.Left, Axes.RightX, 0.0f, true)]
[DefaultStickBinding(StickDirection.Right, Keys.D)]
[DefaultStickBinding(StickDirection.Right, Axes.RightX, 0.0f, false)]
[SettingName("Dash")]
public VirtualStick Dash { get; set; } = new VirtualStick("Dash", VirtualAxis.Overlaps.TakeNewer, 0.35f);