Add language server command to get entry point parameters

[turbolent]

Closes #405

• Refactor the existing entry point analysis (transaction declaration or script function declaration) from the language server to the AST and sema. It could be useful in other tools, and having it in a central place ensures the assumptions won't get out of sync.
• Add a function to sema which returns the parameters of the entry point
• Add a command to the language server which returns the parameters of the entry point, leveraging the function in sema. This allow e.g. the VS Code extension and Playground, which use the language server, to request this information
• Add a test suite for the language server
• Add a test for the new command

[jordanschalm]

👏 Nice work