feat: add rudimentary tab completion for executables to shell

programs/src/shell/Makefile

@@ -64,7 +64,7 @@ LDFLAGS := \
 
 # ---- Source files ----
 
-SRCS := main.cpp vars.cpp builtins.cpp exec.cpp
+SRCS := main.cpp vars.cpp builtins.cpp exec.cpp tabcompletion.cpp
 OBJS := $(addprefix $(OBJDIR)/,$(SRCS:.cpp=.o))
 
 # ---- Target ----