Preprocessors support

### Your environment
Output of `haskell-language-server --probe-tools` or `haskell-language-server-wrapper --probe-tools`:

```sh
haskell-language-server version: 0.7.1.0 (GHC: 8.10.2) (PATH: /home/berberman/.cabal/store/ghc-8.10.2/haskell-language-server-0.7.1.0-1b853bbfb9efed2b28ed42e7c45d0977aeea9a80a3df9b1ce8cdbe96a4efaaf6/bin/haskell-language-server)
Tool versions found on the $PATH
cabal: 3.2.0.0
stack: Not found
ghc: 8.10.2
```
Which lsp-client do you use:

vscode

Describe your project (alternative: link to the project):
in cabal file:
```
build-tools-depends: hsc2hs
```
### Steps to reproduce

Open a `.hsc` file, HLS didn't know it was a haskell source file.

### Expected behaviour

HLS should be able to load `.hsc` (hsc2hs) and `.chs` (c2hs) files.
### Actual behaviour

HLS ignore those files.