Validation

[Rich-Harris]

Should have a validation step that produces errors/warnings as appropriate, before the component is generated. For example, data should always be a function expression, methods shouldn't clash with builtins, computed properties can't have cyclical dependencies, etc etc.

[Rich-Harris]

Basic validation now implemented, so I'll close this. Only validates export default for now, in future we can add more complex template validation (e.g. an <input> with two-way binding can't have a dynamic type attribute, etc etc)