preference: add support for 'files.trimTrailingWhitespace'

[vince-fugnitto]

Feature Description:

The following preference should be included in the framework: files.trimTrailingWhitespace.
The preference is useful to control the automatic trimming of whitespace characters when saving a text document. At the moment, there is no automatic trimming present in the framework.

'files.trimTrailingWhitespace': {
  'type': 'boolean',
  'default': false,
  'description': nls.localize('trimTrailingWhitespace', "When enabled, will trim trailing whitespace when saving a file."),
  'scope': ConfigurationScope.LANGUAGE_OVERRIDABLE
}

Additional Information

• VS Code Implementation

[colin-grant-work]

I'll take a crack at it.