Add initial design files and project configuration for LAB3

- Created a new Block Design Archive (lab_3.bda) for LAB3, defining nodes and edges for the design.
- Added a placeholder README file in the simulation directory.
- Initialized a Vivado project file (lab3.xpr) with configuration settings and source files for synthesis and simulation.
- Updated vhdl_ls.toml to include LAB3 source and simulation files for VHDL language server support.

LAB3/sim/ReadMe.md

@@ -0,0 +1,3 @@
+# Placeholder
+
+This is a placeholder.