feat: add stdlib IO and extern function support

- Add stdlib/io.c with PrintLine, Print, PrintInt, PrintFloat, PrintBool, ReadLine
- Support extern function declarations (functions without body)
- Generate C forward declarations for extern functions
- Map Bux type names (String, int, etc.) to C types in codegen
- Update build system to copy all stdlib files
- Hello World example works: prints 'Hello, Bux!'

src/hir.nim

@@ -126,6 +126,7 @@ type
 
   HirModule* = object
     funcs*: seq[HirFunc]
+    externFuncs*: seq[HirFunc]  # Functions declared with extern (no body)
     structs*: seq[tuple[name: string, fields: seq[tuple[name: string, typ: Type]]]]
     enums*: seq[tuple[name: string, variants: seq[string]]]
     consts*: seq[tuple[name: string, typ: Type, value: HirNode]]