Fix types by moving source-map into dependencies

Closes GH-81.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

package.json

@@ -51,6 +51,7 @@
     "periscopic": "^3.0.0",
     "remark-parse": "^10.0.0",
     "remark-rehype": "^9.0.0",
+    "source-map": "^0.7.0",
     "unified": "^10.0.0",
     "unist-util-position-from-estree": "^1.0.0",
     "unist-util-stringify-position": "^3.0.0",
@@ -96,7 +97,6 @@
     "remark-preset-wooorm": "^9.0.0",
     "rimraf": "^3.0.0",
     "rollup": "^2.0.0",
-    "source-map": "^0.7.0",
     "tape": "^5.0.0",
     "theme-ui": "^0.10.0",
     "type-coverage": "^2.0.0",