feature: create ocamlinterface filetype for mli files

This is needed for supporting the tree-sitter parsers in ocaml because
the interface parser is different from the implementation one one.

Author: undu

README.md

@@ -17,7 +17,7 @@ NeoBundleInstall ocaml/vim-ocaml'
 
 " or use NeoBundleLazy
 NeoBundleLazy 'rgrinberg/vim-ocaml', {'autoload' : {'filetypes' :
-    \ ['ocaml', 'dune', 'opam', 'oasis', 'omake', 'ocamlbuild_tags', 'sexplib']}}
+    \ ['ocaml', 'ocamlinterface', 'dune', 'opam', 'oasis', 'omake', 'ocamlbuild_tags', 'sexplib']}}
 ```
 
 ## History

ftdetect/ocaml.vim

@@ -1 +1 @@
-au BufNewFile,BufRead *.ml,*.mli,*.mll,*.mly,.ocamlinit,*.mlt,*.mlp,*.mlip,*.mli.cppo,*.ml.cppo setf ocaml
+au BufNewFile,BufRead *.ml,*.mll,*.mly,.ocamlinit,*.mlt,*.mlp,*.ml.cppo setf ocaml

ftdetect/ocamlinterface.vim

@@ -0,0 +1 @@
+au BufNewFile,BufRead *.mli,*.mlip,*.mli.cppo setf ocamlinterface

ftplugin/ocamlinterface.vim

@@ -0,0 +1,12 @@
+" Language:    OCaml Interface
+" Maintainer:  vim-ocaml maintainers
+" URL:         http://www.github.com/ocaml/vim-ocaml
+
+if exists("b:did_ftplugin")
+  finish
+endif
+
+runtime! ftplugin/ocaml.vim
+runtime! ftplugin/ocaml_*.vim ftplugin/ocaml/*.vim
+
+" vim:sw=2 fdm=indent