LibJS: Capture UnrealizedSourceRanges in ExecutionContext, not ASTNodes

This loosens the connection to the AST interpreter and will allow us to
generate SourceRanges for the Bytecode interpreter in the future as well

Moves UnrealizedSourceRanges from TracebackFrame to the JS namespace for
this

Userland/Libraries/LibJS/AST.h

@@ -58,6 +58,10 @@ public:
     virtual void dump(int indent) const;
 
     [[nodiscard]] SourceRange source_range() const;
+    UnrealizedSourceRange unrealized_source_range() const
+    {
+        return { m_source_code, m_start_offset, m_end_offset };
+    }
     u32 start_offset() const { return m_start_offset; }
     u32 end_offset() const { return m_end_offset; }