.eslintrc.json: enable comma-spacing plugin for json5 files

We want to avoid this:

  {
    // "bitstream.download.page.back": "Back" ,
    "bitstream.download.page.back": "Atrás" ,
  }

There is no need for the space before the comma. Note that we do
not need this plugin to check for spaces after the comma because
those will be highlighted by the other trailing whitespace checks.

See: https://eslint.org/docs/latest/rules/comma-spacing

.eslintrc.json

@@ -252,6 +252,12 @@
             "maxEOF": 1
           }
         ],
+        "comma-spacing": ["error",
+          {
+            "before": false,
+            "after": true
+          }
+        ],
         "jsonc/comma-dangle": [
           "error",
           "always-multiline"