LibPDF: Implement name tree lookups

Name Trees are hierarchical, string-keyed, sorted-by-key dictionary
structures in PDF where each node (except the root) specifies the bounds
of the values it holds, and either its kids (more nodes) or the
key/value pairs it contains.

This commit implements a series of lookup calls for finding a key in
such name trees. This implementation follows the tree as needed on each
lookup, but if that becomes inefficient in the long run we can switch to
creating a HashMap with all the contents, which as a drawback will
require more memory.

Userland/Libraries/LibPDF/CommonNames.h

@@ -93,12 +93,14 @@
     A(Length1)                    \
     A(Length2)                    \
     A(Length3)                    \
+    A(Limits)                     \
     A(Linearized)                 \
     A(ML)                         \
     A(Matrix)                     \
     A(MediaBox)                   \
     A(MissingWidth)               \
     A(N)                          \
+    A(Names)                      \
     A(Next)                       \
     A(O)                          \
     A(OP)                         \