Resolve "panic when entering "a"" ✨️

Closes #71 🐔️👍️
Approved-by: aalbert <augustin.albert@bleu-azure.fr>

🦀️🍰🦀️🍰🦀️🍰

* feat(proost): add test for is_command behavior with 1 letter

* fix(proost): add missing boundary check

proost/src/main.rs

@@ -78,12 +78,25 @@ fn main() -> Result<'static, ()> {
 }
 
 fn is_command(input: &str) -> bool {
-    input.chars().position(|c| !c.is_whitespace()).map(|pos| input[pos..pos + 2] != *"//").unwrap_or_else(|| false)
+    input
+        .chars()
+        .position(|c| !c.is_whitespace())
+        .map(|pos| input.len() < 2 || input[pos..pos + 2] != *"//")
+        .unwrap_or_else(|| false)
 }
 
 #[cfg(test)]
 mod tests {
 
+    #[test]
+    fn is_command_no_crash() {
+        assert!(!super::is_command(""));
+        assert!(super::is_command("a"));
+        assert!(super::is_command("aa"));
+        assert!(super::is_command("aaa"));
+        assert!(super::is_command("aaaa"));
+    }
+
     #[test]
     fn is_command_false() {
         assert!(!super::is_command("    "));