Running setIsAsync(true) on ArrowFunction incorrectly inserts async keyword

[tmaarse]

Describe the bug

Version: 24.0.0 (latest)

Using setIsAsync(true) on an ArrowFunction (which indirectly calls addModifier) can result in an error due to incorrect insertion of the async keyword.

To Reproduce

Self-contained reproduction file:

import { Node, Project } from "ts-morph";

const project = new Project();
const fileText = `const f = a => a * 2;`;
const sourceFile = project.createSourceFile("main.js", fileText);

sourceFile.forEachDescendant((node) => {
  if (Node.isArrowFunction(node)) {
    node.setIsAsync(true);
  }
});

This results in:

ManipulationError: Manipulation error: A syntax error was inserted.

main.js:1:13 - error TS1005: ',' expected.

1 const f = a async => a * 2;
              ~~~~~
main.js:1:19 - error TS1005: ';' expected.

1 const f = a async => a * 2;
                    ~~

Error replacing tree: The children of the old and new trees were expected to have the same count (2:3).

-- Details --
Path: repro/main.js
Text: "const f = a async => a * 2;"
Stack: Error: Error replacing tree: The children of the old and new trees were expected to have the same count (2:3).
    at #handleChildren (repro/node_modules/ts-morph/dist/ts-morph.js:1469:19)
    at ParentFinderReplacementNodeHandler.handleNode (repro/node_modules/ts-morph/dist/ts-morph.js:1463:33)
    at ParentFinderReplacementNodeHandler.handleNode (repro/node_modules/ts-morph/dist/ts-morph.js:1620:19)
    at doManipulation (repro/node_modules/ts-morph/dist/ts-morph.js:2364:21)
    at insertIntoParentTextRange (repro/node_modules/ts-morph/dist/ts-morph.js:2398:5)
    at ArrowFunction.addModifier (repro/node_modules/ts-morph/dist/ts-morph.js:9175:13)
    at ArrowFunction.toggleModifier (repro/node_modules/ts-morph/dist/ts-morph.js:9152:22)
    at ArrowFunction.setIsAsync (repro/node_modules/ts-morph/dist/ts-morph.js:2884:18)
    at file://repro/arrow.mjs:9:10
    at nodeCallback (repro/node_modules/ts-morph/dist/ts-morph.js:3345:33)
    at throwError (repro/node_modules/ts-morph/dist/ts-morph.js:2380:19)
    at doManipulation (repro/node_modules/ts-morph/dist/ts-morph.js:2374:13)
    at insertIntoParentTextRange (repro/node_modules/ts-morph/dist/ts-morph.js:2398:5)
    at ArrowFunction.addModifier (repro/node_modules/ts-morph/dist/ts-morph.js:9175:13)
    at ArrowFunction.toggleModifier (repro/node_modules/ts-morph/dist/ts-morph.js:9152:22)
    at ArrowFunction.setIsAsync (repro/node_modules/ts-morph/dist/ts-morph.js:2884:18)
    at file://repro/arrow.mjs:9:10
    at nodeCallback (repro/node_modules/ts-morph/dist/ts-morph.js:3345:33)
    at repro/node_modules/ts-morph/dist/ts-morph.js:3386:37
    at VariableDeclaration.forEachChild (repro/node_modules/ts-morph/dist/ts-morph.js:3325:37) {
  filePath: 'repro/main.js',
  oldText: 'const f = a => a * 2;',
  newText: 'const f = a async => a * 2;'
}

Node.js v20.18.0

Expected behavior

I would expect the resulting file to be const f = async a => a * 2;.