LibJS: Change wording of ErrorType::NotA to be independent of context

Currently, we have NotA and NotAn, to be used dependent on whether the
following word begins with a vowel or not. To avoid this, change the
wording on NotA to be independent of this context.

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toFixed.js

@@ -31,7 +31,7 @@ describe("errors", () => {
         [true, [], {}, Symbol("foo"), "bar", 1n].forEach(value => {
             expect(() => Number.prototype.toFixed.call(value)).toThrowWithMessage(
                 TypeError,
-                "Not a Number object"
+                "Not an object of type Number"
             );
         });
     });

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toString.js

@@ -77,7 +77,7 @@ describe("errors", () => {
         [true, [], {}, Symbol("foo"), "bar", 1n].forEach(value => {
             expect(() => Number.prototype.toString.call(value)).toThrowWithMessage(
                 TypeError,
-                "Not a Number object"
+                "Not an object of type Number"
             );
         });
     });

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.valueOf.js

@@ -14,7 +14,7 @@ describe("errors", () => {
         [true, [], {}, Symbol("foo"), "bar", 1n].forEach(value => {
             expect(() => Number.prototype.valueOf.call(value)).toThrowWithMessage(
                 TypeError,
-                "Not a Number object"
+                "Not an object of type Number"
             );
         });
     });