Define proper types for the frontend tableau representation (#79)

I initially distinguished between terminal and internal trees as well
as between terminal nodes, internal nodes and unsupported nodes, but
applying this typing in the logic and convincing tsc that everything
was fine became too unwildly.

Author: jgonggrijp

frontend/src/app/types.ts

@@ -178,3 +178,21 @@ export type ParseResponse = {
     error: string | null;
 };
 
+//
+// Our internal representation of proof trees
+//
+
+export type ProofNode = {
+    id?: number;
+    rule?: string;
+    mod?: string;
+    head?: string;
+    args?: string;
+    sign?: boolean;
+    end?: true;
+};
+
+export interface ProofTree {
+    nodes: ProofNode[];
+    subtrees?: ProofTree[];
+}