Add bare Dune project + a few small changes

.gitignore

+_build/
+_opam/
+*.opam
\ No newline at end of file

bin/client_db.ml

+let () = print_endline "Hello, World!"

bin/dune

+(executable
+ (public_name client_db)
+ (name client_db)
+ (libraries client_db))

dune-project

+(lang dune 3.6)
+
+(name client_db)
+
+(generate_opam_files true)
+
+(source
+ (github username/reponame))
+
+(authors "Author Name")
+
+(maintainers "Maintainer Name")
+
+(license LICENSE)
+
+(documentation https://url/to/documentation)
+
+(package
+ (name client_db)
+ (synopsis "A short synopsis")
+ (description "A longer description")
+ (depends (ocaml (>="5.0.0")) dune)
+ (tags
+  (topics "to describe" your project)))
+
+; See the complete stanza docs at https://dune.readthedocs.io/en/stable/dune-files.html#dune-project

lib/dune

+(library
+ (name client_db))

test/dune

+(test
+ (name client_db))