Update Kotlin version, refine .gitignore, and optimize tests

Upgraded Kotlin Multiplatform plugin to version 2.1.20 for better compatibility and feature enhancements. Adjusted .gitignore to include `.idea/**` for broader IntelliJ configuration exclusion. Increased test iteration count in `TestCachedString` to improve benchmarking precision.

src/jvmTest/kotlin/nl/astraeus/tba/TestCachedString.kt

@@ -25,7 +25,7 @@ class TestCachedString {
     notCachedPerson.name = "A Longer Longer Longer Longer Longer Test"
 
     var total = 0
-    repeat(1000000) {
+    repeat(10000000) {
       total += cachedPerson.name.hashCode()
       total += notCachedPerson.name.hashCode()
     }
@@ -34,14 +34,14 @@ class TestCachedString {
 
     var start1 = System.nanoTime()
     var total1 = 0
-    repeat(1000000) {
+    repeat(10000000) {
       total1 += cachedPerson.name.hashCode()
     }
     val stop1 = System.nanoTime()
 
     var start2 = System.nanoTime()
     var total2 = 0
-    repeat(1000000) {
+    repeat(10000000) {
       total2 += notCachedPerson.name.hashCode()
     }
     val stop2 = System.nanoTime()