Add the 5 modifier jsdoc tags

[sandersn]

@public @private @protected @readonly @override

Three notes:

• I did not add support for BinaryExpressions yet because they don't have a Modifiers list. I think the right thing is to add it (along with a Type node), but I want to bring it up at a design meeting first.
• I dropped the error messages with tag-specific wording for @override. This seems unnecessary to me -- people who use JS as TS are aware that's what they're doing.
• I added a setModifiers method on nodeData and exposed it via a public AsMutable method on Node. I don't know if that's exactly the right way to do it.