Resolved compiler errors
Mostly by removing erroring code

- Updated project to use .NET Standard 2.1, which is what the game uses
- Updated CodeGenerator to use .NET 6

A bunch of errors still

Removed old dependencies, including uREPL
Added new block IDs
Implemented basic command handling to support existing mod commands

Removed all of the different block property getter methods

Except those that actually refer to the game's code

Added some TODOs as well

Removed BlockIdentifiers.OWNED_BLOCKS as the original got replaced with an array
Added the correct group for each supported functional block
Removed EntityFactory property from IEntitySerializer as it is provided on deserialization

Placement, movement, rotation, removal
Block looked at (in Player class), connected blocks

Removed GameState methods from block APIs
Created a BlockUtility class to get the block the player is looking at
Changed the return type of PlaceBlock, returning the ID of the newly placed block or null

Fixed compile issues and added new blocks