docs(rust piscine): Add is_anagram optional exercise subject

subjects/is_anagram/README.md

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+## is_anagram
+
+### Instructions
+
+Write a function called `is_anagram` that checks if one string is an anagram of another string. An anagram is a word or phrase formed by rearranging the letters of another, such as "listen" and "silent."
+
+```rust
+pub fn is_anagram(s1: &str, s2: &str) -> bool {
+	// Your code goes here
+}
+```
+
+- `s1: &str`: The first input string.
+- `s2: &str`: The second input string.
+
+The function should return `true` if `s1` is an anagram of `s2`, and `false` otherwise.
+Your task is to implement the `is_anagram` function to determine whether the two input strings are anagrams of each other.
+
+### Usage
+
+Here is a possible runner to test your function:
+
+```rust
+use is_anagram::is_anagram;
+
+fn main() {
+    let s1 = "listen";
+    let s2 = "silent";
+
+    if is_anagram(s1, s2) {
+        println!("{} and {} are anagrams!", s1, s2);
+    } else {
+        println!("{} and {} are not anagrams.", s1, s2);
+    }
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+listen and silent are anagrams!
+```