simplifiy build; use debhelper compatibility level 12 (not 9); build-depend on debhelper-compat (not autoconf automake cdbs debhelper dh-autoreconf libtool)

- Organize debian/control fields;
- Remove "Suggests: libsass-doc" which has never been packaged for Debian.
- [Not applied]: Bump debhelper compatibility level to 13:
  * Cannot yet use debhelper-compat due to CDBS bug #946677;
  * Cannot yet use debhelper >= 11 due to CDBS bug #885407.

Update to upstream version '3.6.4'
with Debian dir c11609dc4610c6a20dbb2af1fb917a89fdbf0709

Fix some null pointer access crashes

Fixes https://github.com/sass/libsass/issues/3063
Fixes https://github.com/sass/libsass/issues/3035

Ensure correct output order of compound selectors

Fix abspath handling on windows without directory

Add back C-API getters for plugin paths

Fix an interesting memory handling edge case

Fixes https://github.com/sass/libsass/issues/3078

It seems the ternary is returning the type of the second argument.
This means that the first one was first upgraded to a shared object,
and once assigned to a normal pointer, it got prematurely collected.

They are probably not really useful, but we already
have them documented in the context API documentation.

Fixes https://github.com/sass/libsass/issues/3084

Add new libsass spec repository to CI test

https://github.com/mgreter/libsass-spec

Update to upstream version '3.6.3'
with Debian dir 5f5c44189612c1f25de743e9f8699bf9e5bc6e04

* Change "sanctioned" to "approved"

Clarify the meaning of the list in the Readme.md.

* Change "Approved" to "Supported"

Suggestion from #3011.

There are other options in the comments still. I'm not really sure how to come to consensus in an open source project on a qualitative change like this.

To make this project "go gettable".

Clses #3055

Fixes #3056

 Handle loaded source code as shared objects 

Reported in https://oss-fuzz.com/testcase-detail/5739632306421760

Fix parent resolving for selector schema and real parents

When only one complex selector had a real parent reference
we wrongly marked the whole selector list as chrooted.

Fixes https://github.com/sass/libsass/issues/3046

Makes them survive our context more easily. This enables
use to safe and cheaply pass them to error handlers.