feat(js): add solution and documentation for easy challenge 8 - Vowel counter

Author: IbatoLionDev

1-EASY/JavaScript/8-vowel-counter/article.md

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+# Challenge Description and Solution
+
+## English Version
+
+### Challenge Description
+Create a function that counts the number of vowels in a given string. Consider both uppercase and lowercase letters.
+
+### Code Explanation
+The `countVowels` function converts the string to lowercase and then counts how many characters are vowels using a loop and checking membership in the vowels string.
+
+### Relevant Code Snippet
+
+```javascript
+function countVowels(s) {
+    const vowels = "aeiou";
+    s = s.toLowerCase();
+    let count = 0;
+    for (const char of s) {
+        if (vowels.includes(char)) {
+            count++;
+        }
+    }
+    return count;
+}
+```
+
+### Example Usage
+
+```javascript
+const testStrings = ["Hello", "World", "Python", "OpenAI"];
+testStrings.forEach(str => {
+    console.log(`Number of vowels in '${str}': ${countVowels(str)}`);
+});
+```
+
+---
+
+## Versión en Español
+
+### Descripción del Reto
+Crea una función que cuente el número de vocales en una cadena dada. Considera tanto mayúsculas como minúsculas.
+
+### Explicación del Código
+La función `countVowels` convierte la cadena a minúsculas y luego cuenta cuántos caracteres son vocales utilizando un bucle y comprobando la pertenencia en la cadena de vocales.
+
+### Fragmento de Código Relevante
+
+```javascript
+function countVowels(s) {
+    const vowels = "aeiou";
+    s = s.toLowerCase();
+    let count = 0;
+    for (const char of s) {
+        if (vowels.includes(char)) {
+            count++;
+        }
+    }
+    return count;
+}
+```
+
+### Ejemplo de Uso
+
+```javascript
+const testStrings = ["Hello", "World", "Python", "OpenAI"];
+testStrings.forEach(str => {
+    console.log(`Número de vocales en '${str}': ${countVowels(str)}`);
+});

1-EASY/JavaScript/8-vowel-counter/proposed-solution/main.js

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+// Challenge: Create a function that counts the number of vowels in a given string. Consider both uppercase and lowercase letters.
+
+function countVowels(s) {
+    const vowels = "aeiou";
+    s = s.toLowerCase();
+    let count = 0;
+    for (const char of s) {
+        if (vowels.includes(char)) {
+            count++;
+        }
+    }
+    return count;
+}
+
+// Example usage
+const testStrings = ["Hello", "World", "Python", "OpenAI"];
+testStrings.forEach(str => {
+    console.log(`Number of vowels in '${str}': ${countVowels(str)}`);
+});