sync(filepaths): sync editor field when missing (#576)

This fixes a bug that stopped `configlet sync -u` from writing `editor`
filepaths.

So far, only the reasonml track has an `files.editor` key in the track
`config.json`, so this bug hasn't had much impact.

Before this PR:

    $ git clone https://github.com/exercism/reasonml.git
    $ cd reasonml
    $ git checkout 02b6e04f95b00debeab849fee4b11f45c4274733
    $ jq '.files.editor' config.json
    [
      "src/%{pascal_slug}.rei"
    ]
    $ jq '.files' exercises/practice/acronym/.meta/config.json
    {
      "solution": [
        "src/Acronym.re"
      ],
      "test": [
        "__tests__/Acronym_test.re"
      ],
      "example": [
        ".meta/src/Example.re"
      ]
    }
    $ configlet sync --filepaths  -e acronym -uy
    Checking exercises...
    [warn] filepaths: unsynced: acronym
    Updated the filepaths for 1 exercise
    The `acronym` exercise has up-to-date filepaths!
    $ git status
    HEAD detached at 02b6e04
    nothing to commit, working tree clean

With this PR, after the same `configlet sync` command:

    $ git diff
    --- a/exercises/practice/acronym/.meta/config.json
    +++ b/exercises/practice/acronym/.meta/config.json
    @@ -19,6 +19,9 @@
         ],
         "example": [
           ".meta/src/Example.re"
    +    ],
    +    "editor": [
    +      "src/Acronym.rei"
         ]
       },
       "source": "Julien Vanier",

Co-authored-by: ee7 <45465154+ee7@users.noreply.github.com>

Author: ErikSchierboom

src/sync/sync_common.nim

@@ -361,6 +361,11 @@ func filesKeyOrder(val: ConceptExerciseFiles | PracticeExerciseFiles;
       let insertionIndex = result.find(fkTest) + 1
       result.insert(fkEx, insertionIndex)
 
+    # If `editor` is missing and not empty, write it after `example`/`exemplar`.
+    if fkEditor notin result and val.editor.len > 0:
+      let insertionIndex = result.find(fkEx) + 1
+      result.insert(fkEditor, insertionIndex)
+
 func addFiles(s: var string; val: ConceptExerciseFiles | PracticeExerciseFiles;
               prettyMode: PrettyMode; indentLevel = 1) =
   ## Appends the pretty-printed JSON for a `files` key with value `val` to `s`.