feat: restructure Makefile to delegate builds to script/wren.mk and output to bin/ and lib/ directories

The monolithic Makefile is replaced with a thin wrapper that invokes
script/wren.mk for each configuration. Build artifacts are now placed
in bin/ (executables) and lib/ (static/shared libraries) instead of
the project root. The .gitignore is updated accordingly, and the
getting-started documentation reflects the new output layout.

.gitignore

@@ -1,19 +1,12 @@
-# Intermediate files directory.
+# Build outputs.
+bin/
+lib/
+wren
+
+# Intermediate files.
 build/
 .sass-cache/
 
-# Built files at the top level.
-wren
-wrend
-wren-cpp
-libwren.a
-libwrend.a
-libwren.so
-libwrend.so
-libwren.dylib
-libwrend.dylib
-libwren-cpp.a
-
 # XCode user-specific stuff.
 xcuserdata/
 *.xccheckout