Improve source structure in the node api (#6164)

* Update api/node/typescript/models.ts

Co-authored-by: Simon Hausmann <simon.hausmann@slint.dev>

* Code review feedback
---------

Co-authored-by: Simon Hausmann <simon.hausmann@slint.dev>

api/node/rust/interpreter/diagnostic.rs

@@ -0,0 +1,61 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+use slint_interpreter::{Diagnostic, DiagnosticLevel};
+
+/// This enum describes the level or severity of a diagnostic message produced by the compiler.
+#[napi(js_name = "DiagnosticLevel")]
+pub enum JsDiagnosticLevel {
+    /// The diagnostic found is an error that prevents successful compilation.
+    Error,
+
+    /// The diagnostic found is a warning.
+    Warning,
+}
+
+impl From<DiagnosticLevel> for JsDiagnosticLevel {
+    fn from(diagnostic_level: DiagnosticLevel) -> Self {
+        match diagnostic_level {
+            DiagnosticLevel::Warning => JsDiagnosticLevel::Warning,
+            _ => JsDiagnosticLevel::Error,
+        }
+    }
+}
+
+/// This structure represent a diagnostic emitted while compiling .slint code.
+///
+/// It is basically a message, a level (warning or error), attached to a
+/// position in the code.
+#[napi(object, js_name = "Diagnostic")]
+pub struct JsDiagnostic {
+    /// The level for this diagnostic.
+    pub level: JsDiagnosticLevel,
+
+    /// Message for this diagnostic.
+    pub message: String,
+
+    /// The line number in the .slint source file. The line number starts with 1.
+    pub line_number: u32,
+
+    // The column in the .slint source file. The column number starts with 1.
+    pub column_number: u32,
+
+    /// The path of the source file where this diagnostic occurred.
+    pub file_name: Option<String>,
+}
+
+impl From<Diagnostic> for JsDiagnostic {
+    fn from(internal_diagnostic: Diagnostic) -> Self {
+        let (line_number, column) = internal_diagnostic.line_column();
+        Self {
+            level: internal_diagnostic.level().into(),
+            message: internal_diagnostic.message().into(),
+            line_number: line_number as u32,
+            column_number: column as u32,
+            file_name: internal_diagnostic
+                .source_file()
+                .and_then(|path| path.to_str())
+                .map(|str| str.into()),
+        }
+    }
+}