v4 script parser

[SanderMertens]

This PR reimplements the Flecs Script addon. The new implementation:

• Implements a new & improved syntax
• Has a new recursive descent parser that creates an AST
• Has a separate AST visitor that runs the script
• Has a new AST visitor for translating the AST into a string
• Has a more streamlined script API for working with expressions/variables
• Merges the EXPR and SCRIPT addons
• Adds support for if statements
• Moves script test suites into their own project