[compiler-v2][Feature Request] Index notation for tables

[alnoki]

@brmataptos @fEst1ck @georgemitenkov @rahxephon89 @runtian-zhou @vineethk @wrwg

Index notation is already available for vectors and for global resources, but not tables, which would analogously look something like:

• &t[key] for borrow item in the table
• &mut t[key].field for mutably borrow field of table item
• etc.

[wrwg]

We want to look at this once we have trait Indexable or similar.