Add error codes for all errors

[Susurrus]

This is related to work for adding explanations for all errors (#32777) and updating all errors to the new error format (#35233). Unfortunately not all errors have error codes. This is a tracking issue for those errors. I'd think eventually we'd want error codes for all errors so that they're easy to reference.

• Field unknown: attempted access of field _ on type _, but no field with that name was found
• Doesn't live long enough: _ does not live long enough
• Temporary doesn't live long enough: borrowed value does not live long enough
• Better mutability: cannot borrow immutable borrowed content _ as mutable
• Type unknown: the type of this value must be known in this context
• Non-scalar cast: non-scalar cast: _ as _
• No associated item: no associated item named _ found for type _ in the current scope

cc @GuillaumeGomez

[GuillaumeGomez]

Ah nice, I'll work on it then!