[SDL] Add missing modifier keys

[sixfourtwelve]

My character uses LeftShift to sprint, so I added the missing keys in my local fork and included the other keys that I may potentially use

[Copilot]

Pull request overview

This PR adds support for modifier keys (LeftShift, RightShift, LeftControl, RightControl) to the keyboard input system by updating both the core keyboard header and the SDL3 keyboard implementation.

• Adds four new modifier key enums to the Key enumeration
• Implements bidirectional mapping between SDL3 scancodes and the new Key enums

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/core/include/growl/core/input/keyboard.h	Adds four new Key enum values for left/right shift and control modifier keys
src/plugins/sdl3/src/sdl3_keyboard.cpp	Implements SDL3 scancode mapping for the new modifier keys in both getKey() and getScancode() functions

Comments suppressed due to low confidence (1)

src/plugins/sdl3/src/sdl3_keyboard.cpp:378

• The getScancode function is missing a case for Key::Backspace. While Backspace is handled in the getKey function (line 221-222), there is no corresponding case in getScancode. This creates an asymmetry between the two functions - getKey can convert SDL_SCANCODE_BACKSPACE to Key::Backspace, but getScancode cannot convert Key::Backspace back to SDL_SCANCODE_BACKSPACE. This should be added after Key::Escape on line 378 to maintain consistency.

	case Key::Return:
		return SDL_SCANCODE_RETURN;
	case Key::Space:
		return SDL_SCANCODE_SPACE;
	case Key::Escape:
		return SDL_SCANCODE_ESCAPE;

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[JoelOtter]

LGTM, thanks :) Might revisit these later to handle them as actual modifiers but it's an improvement for now!