MISC: refactor weaken effect calculation (#1076)

so far we calculate the effect of weaken in three +1 places
ns.weaken
ns.weakenAnalyze
terminal weaken

and server.weaken where the bn mult is applied

i extracted the logic into a new netscript helper function getWeakenEffect
this gives us one place if we want to change the formula

a side effect i added the server.cpuCores to the terminal weaken to future proof it if the npc server core pr (#963) is merged

src/Server/Server.ts

@@ -141,7 +141,7 @@ export class Server extends BaseServer {
 
   /** Lowers the server's security level (difficulty) by the specified amount) */
   weaken(amt: number): void {
-    this.hackDifficulty -= amt * currentNodeMults.ServerWeakenRate;
+    this.hackDifficulty -= amt;
     this.capDifficulty();
   }