Return promise, resolve tsString output, skip write to file if path is set to null

[scmx]

Hi 👋 ! Nice library. Been using it for a while. Here's some suggested changes:

1. Return promise from storyblokToTypescript node api

Otherwise the node api can't really be used when you can't know when it completes.

2. Change return Promise<string>[] resolve tsString

Change result Promise<void> into Promise<string[]>, returning the tsString output.

3. Skip write file if path null

Skip write the tsString to path location if empty string is passed in as path. Default value is a string so it will continue to save by default.

---

The first change is the most interesting one for me. I am currently working around all three things by:

1. Shelling out to the CLI command instead of using node api
2. Reading the file, doing some changes and writing the file again

[scmx]

FYI: Here's the patch I've doing to the resulting file Maybe some of this should end up in a pull request.

function fixTypesFileInconsistencies(typesFile: string) {
    const lines = fs.readFileSync(typesFile, 'utf-8').split('\n');

    const newLines = lines.flatMap((line) => {
        if (line === 'export interface AssetStoryblok {') {
            return [line, `\tfocus?: null;`, `\tfieldtype?: 'asset';`];
        }
        if (line === `\t_uid: string;`) {
            return [line, `\t_editable?: string;`];
        }
        if (line === `\t[k: string]: any;`) {
            return [];
        }
        return [line];
    });

    fs.writeFileSync(typesFile, newLines.join('\n'));
}

1. Remove the [k: string]: any typing. This typing has caused many issues and we were very happy to get rid of it
2. Add the missing _editable?: string typing for preview mode
3. Add typing of focus and fieldtype. I think I added this because I got an error about these after removing the [k: string]: any