Implement the new devServer.open option

[homer0]

What does this PR do?

homer0/projext#50 added a new option to decided whether you want to open the browser or not when running the dev server. This PR implements the new option.

How should it be tested manually?

Since projext hasn't been released yet, you would first need to change its version to homer0/projext#next.

Now, on your project configuration, add a new option on your dev server settings:

...
devServer: {
  ...,
+  open: false,
},
...

If you run your target, it shouldn't open the browser.

And of course:

yarn test
# or
npm test

[coveralls]

Pull Request Test Coverage Report for Build 158

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 157:	0.0%
Covered Lines:	547
Relevant Lines:	547

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💛 - Coveralls