Add option for setting manual path to Fourmolu binary

[georgefst]

I should probably look in to making this work more like serverExecutablePath, where it's empty by default, and allows expands ~. But I'm not sure where that logic lives - I see no results in the whole codebase when searching for "serverExecutablePath".

[fendor]

serverExecutablePath is a feature implemented by vscode-haskell, not HLS! And the support for ~ is a simple search and replace operation.
Here is where we resolve serverExecutablePath: https://github.com/haskell/vscode-haskell/blob/master/src/hlsBinaries.ts#L136
Examples for resolving ~ and ${HOME}: https://github.com/haskell/vscode-haskell/blob/master/src/utils.ts#L181 and https://github.com/haskell/vscode-haskell/blob/master/src/utils.ts#L188

However, the idea is the same, if the config option is either "" or null, then use the built-in formatter, otherwise use the PATH.

[michaelpj]

Can we ask the user's shell to expand the path? That's the limiting case of what we want, right?

[michaelpj]

Returning to this: I think it's pretty sensible for HLS to expect an actual full path, and for it to be the job of the client to shell-expand paths or whatever. So I think the current implementation of this seems fine.

Should we update and merge this?

[georgefst]

Yeah, sure. If anyone really wants to add expansion then there's nothing lost by that being a second PR.

[soulomoon]

LGTM, maybe we can update merge this.