Prevent unrelated casts using `as`

Currently, `SyntaxProtocol` has an `as` method that allows casting to any other syntax node type. Because of this, it’s possible to e.g. cast a `FunctionDeclSyntax` to an `IdentifierExprSyntax` without any compiler errors or warnings, even though we know that the cast will always fail.

What we should do instead, is to only have a `as(_: ExprSyntaxProtocol.Type)` function on `ExprSyntax` (analogous for `StmtSyntax` and the other base nodes).

The `as` function on `SyntaxProtocol` can then be marked as deprecated and produce a warning that the cast will always fail.

`is` and `cast` will need to be updated accordingly.