language: Change extern type syntax

Use a '$' instead of a '.' to indicate a type provided in application
code.

The reason for the change is to have a consistent "extern" symbol that
isn't widely used elsewhere and isn't ambiguous in expressions.

blueprintcompiler/language/common.py

@@ -20,7 +20,13 @@
 
 from .. import gir
 from ..ast_utils import AstNode, validate, docs
-from ..errors import CompileError, MultipleErrors
+from ..errors import (
+    CompileError,
+    MultipleErrors,
+    UpgradeWarning,
+    CompileWarning,
+    CodeAction,
+)
 from ..completions_utils import *
 from .. import decompiler as decompile
 from ..decompiler import DecompileCtx, decompiler