Introduce C library for Pkl

This introduces native C bindings for Pkl.

This uses native-image to generate a C library for Pkl.
This generated library from native-image is wrapped with our own library,
in `pkl.h`.

This produces a static and a dynamic library for each os/arch variant
that Pkl currently supports.

Co-authored-by: Dan Chao <daniel.h.chao@gmail.com>

Author: KushalP

.circleci/config.pkl

@@ -21,7 +21,7 @@ import "jobs/DeployJob.pkl"
 import "jobs/GradleCheckJob.pkl"
 import "jobs/SimpleGradleJob.pkl"
 
-local prbJobs: Listing<String> = gradleCheckJobs.keys.toListing()
+local prbJobs: Listing<String> = (gradleCheckJobs.keys + buildNativeJobs.keys.filter((it) -> it.startsWith("libpkl") && it.endsWith("snapshot"))).toListing() 
 
 local buildAndTestJobs = (prbJobs) {
   "bench"
@@ -81,14 +81,15 @@ triggerPackageDocsBuild = "release"
 
 local buildNativeJobs: Mapping<String, BuildNativeJob> = new {
   for (_dist in List("release", "snapshot")) {
-    for (_project in List("pkl-cli", "pkl-doc")) {
+    for (_project in List("pkl-cli", "pkl-doc", "libpkl")) {
       for (_arch in List("amd64", "aarch64")) {
         for (_os in List("macOS", "linux")) {
           ["\(_project)-\(_os)-\(_arch)-\(_dist)"] {
             arch = _arch
             os = _os
             isRelease = _dist == "release"
             project = _project
+            buildDir = if (_project == "libpkl") "libpkl/build/native-libs/" else "\(_project)/build/executable/"
           }
         }
       }
@@ -98,12 +99,14 @@ local buildNativeJobs: Mapping<String, BuildNativeJob> = new {
         musl = true
         isRelease = _dist == "release"
         project = _project
+        buildDir = if (_project == "libpkl") "libpkl/build/native-libs/" else "\(_project)/build/executable/"
       }
       ["\(_project)-windows-amd64-\(_dist)"] {
         arch = "amd64"
         os = "windows"
         isRelease = _dist == "release"
         project = _project
+        buildDir = if (_project == "libpkl") "libpkl/build/native-libs/" else "\(_project)/build/executable/"
       }
     }
   }