Build AST nodes for module function declarations (#239)

Adds enough to build the AST nodes for the module function declarations in a .em file.

Adds:

- Function: base class for module function declarations and user functions
- FunctionParameterDeclaration: declaration for a function parameter, as well as its default value if any
- FunctionParameterList: just a holder for function parameter declarations
- ModuleFunctionDeclaration: for function declarations in .em files
- FunctionResolver: hooks up function calls to module functions or user functions
- ModuleProcessor: a visitor for processing const declarations and module function declarations (const declarations are yet to come from the main branch)

pol-core/bscript/compiler/ast/ModuleFunctionDeclaration.cpp

@@ -0,0 +1,30 @@
+#include "ModuleFunctionDeclaration.h"
+
+#include <format/format.h>
+#include <utility>
+
+#include "compiler/ast/FunctionParameterDeclaration.h"
+#include "compiler/ast/FunctionParameterList.h"
+#include "compiler/ast/NodeVisitor.h"
+
+namespace Pol::Bscript::Compiler
+{
+ModuleFunctionDeclaration::ModuleFunctionDeclaration(
+    const SourceLocation& source_location, std::string module_name, std::string name,
+    std::unique_ptr<FunctionParameterList> parameter_list )
+    : Function( source_location, std::move( name ), std::move( parameter_list ) ),
+      module_name( std::move( module_name ) )
+{
+}
+
+void ModuleFunctionDeclaration::accept( NodeVisitor& visitor )
+{
+  visitor.visit_module_function_declaration( *this );
+}
+
+void ModuleFunctionDeclaration::describe_to( fmt::Writer& w ) const
+{
+  w << "module-function-declaration(" << module_name << "::" << name << ")";
+}
+
+}  // namespace Pol::Bscript::Compiler