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-executor/pkl-executor.gradle.kts

@@ -16,8 +16,8 @@ dependencies {
 
   implementation(libs.slf4jApi)
 
-  testImplementation(project(":pkl-commons-test"))
-  testImplementation(project(":pkl-core"))
+  testImplementation(projects.pklCommonsTest)
+  testImplementation(projects.pklCore)
   testImplementation(libs.slf4jSimple)
 }