feat: implement scripting API access to user's tile size preferences

[abhinavs1920]

Resolves Issue #4222

This PR exposes user tile size preferences to the scripting API, allowing scripts to read and modify the default tile dimensions used in the New Map and New Tileset dialogs.

Scripts can now access these preferences through tiled.preferences:

mapTileWidth / mapTileHeight - default tile size for new maps
tilesetTileWidth / tilesetTileHeight - default tile size for new tilesets
The implementation adds a new ScriptPreferences class that wraps the existing session options, making them accessible to the scripting engine. Session options that were previously scoped locally in dialog files have been moved to a shared header (sessionoptions.h) so they can be accessed from the scripting layer.

This is useful for scripts that want to create maps or tilesets programmatically using the user's preferred tile dimensions, or for scripts that need to temporarily adjust these defaults.

I am also adding the screenshot to show successful working of the feature.

[abhinavs1920]

Hey @bjorn, I’ve added the PR. I just realized someone else has been assigned to it, if it’s not mergeable, I’d really appreciate any feedback. Thanks!