fix: fix 3 typos and code bugs

src/assignment.lua

@@ -60,7 +60,7 @@ function Assignment:getParams()
 end
 
 function Assignment:del(name)
-  this.data[name] = nil
+  self.data[name] = nil
 end
 
 function Assignment:toString()

src/interpreter.lua

@@ -72,7 +72,7 @@ end
 
 function Interpreter:hasOverride(name)
   local overrides = self:getOverrides()
-  return overrides ~= nill and overrides[name] ~= nil
+  return overrides ~= nil and overrides[name] ~= nil
 end
 
 function Interpreter:evaluate(planoutCode)

src/lib/utils.lua

@@ -33,7 +33,7 @@ local round = function(num, idp)
 end
 
 local deepcopy
--- Delcaration must be separated from definition because it is recursive
+-- Declaration must be separated from definition because it is recursive
 deepcopy = function(orig)
   local copy
   local status, err = pcall(function()