Trim leading/trailing whitespace from fileHeaderComment

[liamnichols]

While testing 0.1 on some existing projects, I found a slight issue when using fileHeaderComment with multiline strings in yaml.

Lets say that we define the following configuration:

fileHeaderComment: |
  // Generated by Create API
  // https://github.com./CreateAPI/CreateAPI
  //
  // swiftformat:disable all
access: public
# ...

You'd expect it to generate some code like this:

// Generated by Create API
// https://github.com./CreateAPI/CreateAPI
//
// swiftformat:disable all

import Foundation 

// ...

However it will actually include two lines between the comment and the import statement because Yaml seems to add a trailing whitespace after the multiline string.

To address this, I've updated the generator so that it trims whitespace and newlines from the generated output. I also added a dedicated test to validate the behaviour since there wasn't one before.