Downgrade yn due to node engine (#942)

package.json

@@ -83,6 +83,6 @@
     "diff": "^4.0.1",
     "make-error": "^1.1.1",
     "source-map-support": "^0.5.6",
-    "yn": "^4.0.0"
+    "yn": "3.1.1"
   }
 }

src/index.ts

@@ -1,6 +1,6 @@
 import { relative, basename, extname, resolve, dirname, join } from 'path'
 import sourceMapSupport = require('source-map-support')
-import * as yn from 'yn'
+import * as ynModule from 'yn'
 import { BaseError } from 'make-error'
 import * as util from 'util'
 import * as _ts from 'typescript'
@@ -26,6 +26,14 @@ declare global {
  */
 export const INSPECT_CUSTOM = util.inspect.custom || 'inspect'
 
+/**
+ * Wrapper around yn module that returns `undefined` instead of `null`.
+ * This is implemented by yn v4, but we're staying on v3 to avoid v4's node 10 requirement.
+ */
+function yn (value: string | undefined) {
+  return ynModule(value) ?? undefined
+}
+
 /**
  * Debugging `ts-node`.
  */