Add F2 screenshot functionality

[itsRevela]

Description

- Adds in-game screenshot support using F2, matching Java Edition behavior (with local-only chat message). Screenshots are saved as PNG files to a `screenshots` folder in the game directory.

Changes

Previous Behavior

- There was no way to take in-game screenshots. To take a screenshot previously, you had to rely on Win+Shift+s or other something like the print screen button or other external tools.

Root Cause

- Screenshot functionality was never implemented in the LCE codebase. Didn't exist.

New Behavior

- Pressing F2 captures the current frame and saves it as a timestamped PNG file (ex: `2026-03-15_14.30.45.png`) in a `screenshots` directory, similar to how Java does it.

Fix Implementation

- Added `MINECRAFT_ACTION_SCREENSHOT` to `EControllerActions` in `App_enums.h`
- Mapped F2 to `KEY_SCREENSHOT` in `KeyboardMouseInput.h`
- Added screenshot capture logic in `Minecraft.cpp`:
    - `run_middle()` listens for the screenshot key press
    - `tick()` reads the framebuffer via `glReadPixels`, flips the image vertically, and writes it as PNG using `stb_image_write.h`
- Included `stb_image_write.h` (library by Sean Barrett) for PNG encoding

AI Use Disclosure

- No AI was used to write the code in this PR.

Related Issues

- None

[Patoke]

This functionality has existed within the game, the only difference is that it saves to the d: drive and uses a local index for the current screenshot instead of its date

This issue has been fixed in https://github.com/Patoke/4JLibs/ which is expected to be implemented into the main project soon

Unfortunately, this would make your commit unmergable as it's overlapping in functionality within the work plan of the project

What you could do is just commit a change that calls C4JRender::DoScreenGrabOnNextPresent() on F2 so that at least this can be merged and you can be added to the contributors list instead of being straight up closed, as well as keeping your other changes

If you need extra functionality for this, please let me know and I can add it to the 4JLibs repository, like a function that returns the name of the last created screenshot or ways to change the output directory

Thanks for your contribution!

[itsRevela]

@Patoke Ah, I understand. I've updated the commit. Feel free to take a look!