feat: implement stringify API with extension points (#3)

src/LineWriter.ts

@@ -0,0 +1,21 @@
+/**
+    Copyright 2022 Google LLC
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+        https://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+ */
+
+export interface LineWriter {
+  appendLine(line: string): LineWriter;
+  startBlock(): LineWriter;
+  endBlock(): LineWriter;
+}

src/LineWriterImpl.ts

@@ -0,0 +1,50 @@
+/**
+    Copyright 2022 Google LLC
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+        https://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+ */
+
+import { LineWriter } from "./LineWriter.js";
+
+export class LineWriterImpl implements LineWriter {
+  #indentation: string;
+  #currentIndentation = 0;
+  #lines: string[] = [];
+
+  constructor(indentation: string) {
+    this.#indentation = indentation;
+  }
+
+  appendLine(line: string): LineWriter {
+    const indentedLine = line
+      ? this.#indentation.repeat(this.#currentIndentation) + line.trimEnd()
+      : "";
+    this.#lines.push(indentedLine);
+    return this;
+  }
+
+  startBlock(): LineWriter {
+    this.#currentIndentation++;
+    return this;
+  }
+
+  endBlock(): LineWriter {
+    this.#currentIndentation--;
+    return this;
+  }
+
+  toString(): string {
+    // Scripts should end with a final blank line.
+    return this.#lines.join("\n") + "\n";
+  }
+}