Add new filter option: `requestheader`

Related issue:
https://github.com/uBlockOrigin/uAssets/issues/32194#issuecomment-4060568820

Usage of `requestheader=` option is exactly same as current `header=`
option. `header=` option has been renamed `responseheader=` and the
current `header=` option is now an alias for `responseheader=`.

Filter authors must cease using `header=` option and instead use the
more accurate `responseheader=` option when uBO 1.71.0 is released.

uBOL for Chromium does support conversion of `requestheader` to a DNR
`requestHeaders` condition. Currently other browser platforms do not
support `requestHeaders` condition.