enh: replace type of ext by specific string

[haecheonlee]

It looks better to have specific string instead of general string for the variable, ext. As well, I removed the duplicate ext logic inside the function, createFile, and replaced it by the parameter.

[The-Code-Monkey]

@haecheonlee can change the string of .tsx | .astro into a type so it can be used in multiple places rather then retyping stuff all over the place.

[haecheonlee]

@The-Code-Monkey Do you have suggestion about how to manage types? I don't see any file to manage types only.

I created a type folder, and added a new type, Extension.

[typicode]

Thanks!