Add joypad input map conversion to project converter

[jpcerrone]

Fixes godotengine#77314

Changes

• Converts all of the Godot 3 joypad button index mappings to their Godot 4 equivalent mappings. "button_index":10 -> "button_index":4
• On Godot 4, the entries for the L2 and R2 buttons have been moved to the axis mappings, so entries for button index mappings for those buttons get converted to axis mappings.
  InputEventJoypadButton, "button_index":6,"pressure":0.0,"pressed":false -> InputEventJoypadMotion, "axis":4,"axis_value":1.0
• Converts the Godot 3 axis mappings for L2 and R2 to the Godot 4 axis mappings . (These are the only 2 axis mappings that have changed).
  "axis":7 -> "axis":5

Testing

Tested by running the converter and the validator with a Godot 3 project that contains a project file with the following entries:

• Button mapping that should be converted to its Godot 4 value.
• Button mapping that shouldn't be converted (Same mapping in 3&4)
• Axis that should be converted. (L2 and R2).
• Axis that shouldn't be converted.
• Button index that's greater than the ammount of mappings (23).

All of these conversion get performed as expected.
Also 3 validation tests have been added in the code for the most general conversion cases.

[akien-mga]

Didn't test, but looks good overall.

[akien-mga]

Thanks!