chore: update assertions to use assert.isString for Project Euler (freeCodeCamp#60601)

Author: cstidm2

curriculum/challenges/english/18-project-euler/project-euler-problems-201-to-300/problem-220-heighway-dragon.md

@@ -28,7 +28,7 @@ What is the position of the cursor after ${10}^{12}$ steps in $D_{50}$? Give you
 `heighwayDragon()` should return a string.
 
 ```js
-assert(typeof heighwayDragon() === 'string');
+assert.isString(heighwayDragon());
 ```
 
 `heighwayDragon()` should return the string `139776,963904`.

curriculum/challenges/english/18-project-euler/project-euler-problems-201-to-300/problem-236-luxury-hampers.md

@@ -36,7 +36,7 @@ What's the largest possible value of $m$? Give your answer as a string with frac
 `luxuryHampers()` should return a string.
 
 ```js
-assert(typeof luxuryHampers() === 'string');
+assert.isString(luxuryHampers());
 ```
 
 `luxuryHampers()` should return the string `123/59`.

curriculum/challenges/english/18-project-euler/project-euler-problems-201-to-300/problem-284-steady-squares.md

@@ -21,7 +21,7 @@ Find the sum of the digits of all the $n$-digit steady squares in the base 14 nu
 `steadySquares()` should return a string.
 
 ```js
-assert(typeof steadySquares() === 'string');
+assert.isString(steadySquares());
 ```
 
 `steadySquares()` should return the string `5a411d7b`.