The VCL_REGEX type requires that the pattern consist of constant strings only.

In contrast, with a STRING argument and compilation in this vmod, we can support
patterns to be constructed dynamically in vcl_init {}, which is a relevant
advantage.

I did consider to support both a STRING and a VCL_REGEX argument (both being
optional, requiring at least one to be set) but found no compelling reason
for using the VCL_REGEX type, because the pattern is checked anyway and it
does not make a relevant difference if that check happens at the VCC or
the vcl_init stage.

This reverts commit 0ee2f4ae.