[Kotlin][FlexBuffers] Add support for Kotlin-JS (#6554)

Flexbuffers for Kotlin currently supports JVM and MacOS. This change
introduces support to JS as well.

kotlin/flatbuffers-kotlin/build.gradle.kts

@@ -8,6 +8,16 @@ version = "1.12.0-SNAPSHOT"
 kotlin {
   explicitApi()
   jvm()
+  js {
+    browser {
+      binaries.executable()
+      testTask {
+        useKarma {
+          useChromeHeadless()
+        }
+      }
+    }
+  }
   macosX64()
 
   sourceSets {
@@ -35,6 +45,15 @@ kotlin {
       }
     }
 
+    val jsMain by getting {
+      dependsOn(commonMain)
+    }
+    val jsTest by getting {
+      dependsOn(commonTest)
+      dependencies {
+        implementation(kotlin("test-js"))
+      }
+    }
     val nativeMain by creating {
         dependsOn(commonMain)
     }
@@ -55,6 +74,7 @@ kotlin {
    *  https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#setting-up-targets */
   targets {
     targetFromPreset(presets.getAt("jvm"))
+    targetFromPreset(presets.getAt("js"))
     targetFromPreset(presets.getAt("macosX64"))
   }
 }