[Botskill] Add validation for .lu and .dispatch file's path

[dfavretto]

Description

Related issue #1246

• Add validation for .lu file, throw Error if file does not exist, stop command execution.
• Add validation for .dispatch file, throw Error if file does not exist, stop command execution.'

Testing Steps

1. Go to AI\lib\typescript\botskills\.
2. Open a terminal in that location.
3. Run the command npm install to install dependencies.
4. Run the command npm run build to build the project.
5. Run the command npm link to symlink the package folder.
6. Run the command botskills connect with the proper arguments, but leading to a nonexistent .lu file.
7. Check the error thrown for the nonexistent .lu file, and the execution of the command is stopped.
8. Repeat the execution of the command botskills connect but this time with a misleading .dispatch path.
9. Check the error thrown for the nonexistent .dispatch file, and the execution of the command is stopped.

Checklist

• I have commented my code, particularly in hard-to-understand areas