Bump fourmolu to 0.19 (#2577)

https://hackage.haskell.org/package/fourmolu-0.19.0.0/changelog

Author: xsebek

.github/restyled.yaml

@@ -2,4 +2,4 @@ restylers_version: stable
 restylers:
   - fourmolu:
       image:
-        tag: v0.18.0.0
+        tag: v0.19.0.0

CONTRIBUTING.md

@@ -215,15 +215,15 @@ them!
 
 #### Formatting style
 
-We use [`fourmolu-0.18.0.0`](https://hackage.haskell.org/package/fourmolu)
+We use [`fourmolu-0.19.0.0`](https://hackage.haskell.org/package/fourmolu)
 with a [custom
 configuration](https://github.com/swarm-game/swarm/blob/main/fourmolu.yaml)
 for formatting Haskell code.
 
 To install the formatter, run:
 
 ```bash
-cabal install fourmolu-0.18.0.0
+cabal install fourmolu-0.19.0.0
 ```
 
 If this installation does not work, you may have to set your GHC to a version supported by `fourmolu`: