Add Racket tree-sitter support, remove Kotlin

- Add tree-sitter-racket dependency (v0.24)
- Initialize Racket parser in code search
- Add .rkt, .rktl, .rktd file extensions
- Add test_racket_search test
- Remove Kotlin from supported languages (was disabled)
- Clean up duplicate test files

Supported languages: Rust, Python, JavaScript, TypeScript, Go, Java, C, C++, Racket

examples/test_code/example.rkt

@@ -0,0 +1,24 @@
+#lang racket
+
+(define (greet name)
+  (printf "Hello, ~a!\n" name))
+
+(define (add x y)
+  (+ x y))
+
+(define (factorial n)
+  (if (<= n 1)
+      1
+      (* n (factorial (- n 1)))))
+
+(struct person (name age) #:transparent)
+
+(define (person-greet p)
+  (printf "Hello, I'm ~a\n" (person-name p)))
+
+(greet "World")
+(displayln (add 5 3))
+(displayln (factorial 5))
+
+(define alice (person "Alice" 30))
+(person-greet alice)