Fix HasteImpl Regex (#24628)

Summary:
The jest HasteImpl's `pluginNameReducers` regex was not properly escaping a backslash, resulting in unintended behavior.

The intent of the code is to strip the `.${name}` from the end of a filepath, where `name` is a platform name. The correct regex for that would be `^(.*)\.(myPlat)$`, but because the regex is being constructed from a string, the `\.` is being interpreted as an escaped period, resulting in the regex  `^(.*).(myPlat)$`. To correct this, the backslash needs to be escaped so it makes it into the regex.

[General] [Fixed] - Fix HasteImpl platform name regex
Pull Request resolved: #24628

Differential Revision: D15224468

Pulled By: hramos

fbshipit-source-id: 6eb507aa5410bdd7c247e6d301052d41995a2f11

Author: CaptainNic

jest/hasteImpl.js

@@ -51,7 +51,7 @@ const pluginRoots /*: Array<
 const pluginNameReducers /*: Array<
   [RegExp, string],
 > */ = plugins.haste.platforms.map(name => [
-  new RegExp(`^(.*)\.(${name})$`),
+  new RegExp(`^(.*)\\.(${name})$`),
   '$1',
 ]);