Some parts of syntax nodes are still typed writable

[nguerrera]

We did https://github.com/Azure/cadl-azure/issues/625, but we still have some writable properties, for example:

• parent
• Also Sym needs to be deeply readonly too since it hangs off of syntax nodes. We will need to be able reuse the result of binding too.

As we keep growing the amount of work we do in the language server, it will be important to reuse syntax trees (https://github.com/Azure/cadl-azure/issues/619), and it will be important that we have some type checking that you're not mutating them outside of binding.

[nguerrera]

I'm prioritizing this because we've added a lot more things that happen on each keystroke.

[nguerrera]

It seems we are still making mutations to CadlScriptNode and NamespaceStatementNode to handle usings.