Reduce the duplicated codes in `rendercode.ts`

[sij411]

There are multiple ways to reduce the duplicated codes:

1. To separate that patterns and have 3 functions (render(), renderRequest(), renderResponse())
2. To combine two functions and have if conditions to process each type Request|Response.

Or other ways

Originally posted by @sij411 in https://github.com/fedify-dev/fedify/pull/410#discussion_r2331995381