Parse dune variable expansions

Nobody has claimed this yet.

I've been trying to use RLS while working on the dune project with each release. Comments work now, but there still seem to be some issues with parsing the dune files. I get the following error on opening the project now:

```
Unable to parse build file /home/zach/src/dune/src/jbuild
Unexpected char: % at 356
```

This is the contents of the dune file at path `~/src/dune/src/dune`:

```dune
(library
(name dune)
(libraries unix
stdune
fiber
xdg
dune_re
threads
opam_file_format
dune_lang
ocaml_config
which_program)
(synopsis "Internal Dune library, do not use!")
(preprocess (action
(run %{project_root}/src/let-syntax/pp.exe %{input-file}))))

(ocamllex meta_lexer glob_lexer dune_lexer)

(rule
(targets setup.ml)
(mode fallback)
(action (copy# setup.defaults.ml setup.ml)))
```

Looks like we're not yet handling the [variable expansions](https://dune.readthedocs.io/en/latest/dune-files.html?highlight=variable#variables-expansion) of the form `%{variable}`.

Start with the reported src/dune example and the linked Dune variable-expansion documentation, then reproduce the unexpected '%' at position 356 while opening the project. Done means Dune files containing %{variable} expansions parse without this error.

Written by the indexing model from the issue text.