napi: Fix access to diagnostics

Invoking the diagnostics getter would throw an exception because the JsDiagnostic type has no constructor.

There's however no constructor that would allow for the previous
laziness, so instead this PR bites the bullet:

- When diagnostics are requested, we convert them all in one go into proper JavaScript objects.
- The vector returned by line_column is replaced with more idiomatic column and lineNumber properties

api/napi/__test__/compiler.spec.ts

@@ -196,3 +196,20 @@ test('globalCallbacks ComponentDefinition', (t) => {
   t.is(callbacks![0], "first-callback");
   t.is(callbacks![1], "second-callback");
 })
+
+test('compiler diagnostics', (t) => {
+  let compiler = new ComponentCompiler;
+  t.is(compiler.buildFromSource(`export component App {
+    garbage
+  }`, "testsource.slint"), null);
+
+  const diags = compiler.diagnostics;
+  t.is(diags.length, 1);
+  t.deepEqual(diags[0], {
+    level: 0,
+    message: 'Parse error',
+    lineNumber: 2,
+    column: 12,
+    sourceFile: 'testsource.slint'
+  });
+})