LibPDF: Add smoke-and-mirror implementation of SeparationColorSpace

None of the methods actually do anything, but we now create an
actual SeparationColorSpace object for /Separation color spaces.

This fixes a crash on page 810 of pdf_reference_1-7.pdf.
Previously, we'd log a "separation color space not supported" error,
which would lead to Renderer not updating its current color space.
It'd stay a DeviceCYMK color space, which would then later assert
when it got a 1-argument array as color (which now the
SeparationColorSpace gets instead, which logs an "unimplemented"
error for that instead of asserting).

Userland/Libraries/LibPDF/CommonNames.h

@@ -134,6 +134,7 @@
     A(SA)                         \
     A(SM)                         \
     A(SMask)                      \
+    A(Separation)                 \
     A(StmF)                       \
     A(StrF)                       \
     A(Subject)                    \