fix: make webpack work on Windows

[alexander-fenster]

Turns out webpack rules break on Windows because of different path separator used. This PR fixes #1732 by fixing the regular expression in the null-module rules in webpack.config.js to accept both / and \ as a possible path separators.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[alexander-fenster]

@JustinBeckwith @bcoe The CI fails for some totally unrelated reason:

  1) Clients
       should include default params when only callback is provided to API call:
     FetchError: request to https://datastore.googleapis.com/v1/projects/test-project-id:lookup?projectId=test-project-id&myParam=123 failed, reason: Nock: No match for request {
  "method": "POST",
  "url": "https://datastore.googleapis.com/v1/projects/test-project-id:lookup?projectId=test-project-id&myParam=123",
  "headers": {
    "accept-encoding": [
      "gzip"
    ],
    "user-agent": [
      "google-api-nodejs-client/2.0.2 (gzip)"
    ],
    "accept": [
      "application/json"
    ],
    "content-length": [
      "0"
    ],
    "connection": [
      "close"
    ]
  }
}
      at OverriddenClientRequest.<anonymous> (node_modules/node-fetch/lib/index.js:1455:11)
      at /tmpfs/src/github/google-api-nodejs-client/node_modules/nock/lib/request_overrider.js:221:11
      at process._tickCallback (internal/process/next_tick.js:61:11)

Do you have an idea what's going on? It obviously cannot be related to my webpack config change in this PR.

[codecov]

Codecov Report

Merging #1742 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1742   +/-   ##
=======================================
  Coverage   96.49%   96.49%           
=======================================
  Files           2        2           
  Lines         428      428           
  Branches        7        7           
=======================================
  Hits          413      413           
  Misses         15       15

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