Add Placeholder Variable for missingTranslationString

[bartholomej]

Feature Description

I've implemented a new enhancement for the missingTranslationString option that allows using a {{t}} placeholder variable to include the translation key itself in the default text.

How it works

When a missing translation is added to language files, you can now use the {{t}} placeholder in the missingTranslationString value to have it replaced with the actual translation key. For example:

• Using --missingTranslationString='Translation missing: {{t}}' would create entries like:
• For key user.greeting: "user.greeting": "Translation missing: user.greeting"
• For key errors.notFound: "errors.notFound": "Translation missing: errors.notFound"

Benefits

1. Provides more contextual default translations
2. Helps developers identify which key needs translation
3. Makes the system more flexible for different use cases
4. Can be used in combination with other text to create meaningful fallback messages

Documentation

I've updated the documentation to include this new functionality in the missingTranslationString section with examples of how to use it.

[bartholomej]

Corrected the module field in package.json from .js to .mjs to match the actual file in the dist folder. The original extension was probably a typo as no .js file exists in dist, only .mjs.