(fluent) Parse variant keys in a strict way

Author: stasm

fluent-syntax/test/fixtures_structure/variant_keys.ftl

@@ -0,0 +1,61 @@
+key01 = { $sel ->
+    *[
+        key
+    ] Value
+}
+
+key02 = { $sel ->
+    *[
+        key
+    ]
+    
+    Value
+}
+
+err01 = { $sel ->
+    *["key"] Value
+}
+
+err02 = { $sel ->
+    *[-key] Value
+}
+
+err03 = { $sel ->
+    *[-key.attr] Value
+}
+
+err04 = { $sel ->
+    *[-key()] Value
+}
+
+err05 = { $sel ->
+    *[-key.attr()] Value
+}
+
+err06 = { $sel ->
+    *[key.attr] Value
+}
+
+err07 = { $sel ->
+    *[$key] Value
+}
+
+err08 = { $sel ->
+    *[FUNC()] Value
+}
+
+err09 = { $sel ->
+    *[{key}] Value
+}
+
+err10 = { $sel ->
+    *[{"key"}] Value
+}
+
+err11 = { $sel ->
+    *[{3.14}] Value
+}
+
+err12 = { $sel ->
+    *[{$key}] Value
+}