Add missing import / export diagnostics

[kaleidawave]

There are a few missing diagnostics for some things related to imports/exports

• export in invalid location
• exporting same name
• Some others that have todo! in features/modules.rs

Would be good to get these wired up similar to other diagnostics

Some of these might be built into the parser at some point

[NitantP]

Hi, @kaleidawave! I took a stab at this in #192, but could use some direction as I'm still learning the codebase.