Add amber as submodule at vendor/amber

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "vendor/amber"]
+	path = vendor/amber
+	url = https://github.com/amber-smalltalk/amber

vendor/amber

@@ -0,0 +1 @@
+Subproject commit 6f77c59f2d960ffc96c2626d12d150b77538584c