Use Gradle typed project accessors

This change activates the `TYPESAFE_PROJECT_ACCESSORS` feature
preview in Gradle, and switches to such accessors instead of
string-based project references, where possible

Relates-To: apple/pkl#204
Signed-off-by: Sam Gammon <sam@elide.ventures>

pkl-commons-test/pkl-commons-test.gradle.kts

@@ -11,7 +11,7 @@ dependencies {
   api(libs.junitApi)
   api(libs.junitEngine)
   api(libs.junitParams)
-  api(project(":pkl-commons")) // for convenience
+  api(projects.pklCommons) // for convenience
   implementation(libs.assertj)
 }