Obsolete `postprocessor` argument of `DtsContent#writeFile`

[mizdra]

• There is a postprocessor option to convert the final output.
  • https://github.com/mizdra/checkable-css-modules/blob/067e89017f6f42fe0b27718735ec7c02c7e3df26/src/dts-content.ts#L95
• This was originally added to format the output by prettier.
  • https://github.com/Quramy/typed-css-modules/pull/83
• However, this option does not support sourcemap.
  • Therefore, it cannot be used with declarationMap. (This is because the mapping of code positions before and after processing with postprocessor is no longer possible and will be broken.)
  • https://github.com/mizdra/checkable-css-modules/blob/067e89017f6f42fe0b27718735ec7c02c7e3df26/src/dts-content.ts#L99-L101
• Therefore, I will obsolete this option
• Instead, I will implement a mechanism to insert a comment at the beginning, such as // prettier-ignore.
  • This will be discussed in a separate issue.