Bug fixes and better checking for invalid syntax

• 417951d: Add VRC namespaces to template script for easier access to PlayerApi and other VRC-related stuff
• 23bc695: Add more specific exceptions for when an operator cannot be found given two operand types
• eca8bb7: Add checking for attempting to set a readonly property
• 4c94137: Fix for handling constructors on structs with more than 0 parameters that aren't defined in Udon
• 4c94137: Add type validation to make sure Udon defines a type before the user can define a variable of that type. This prevents the assembly from silently failing to assemble when the user defines an unsupported variable type.