Gulp build improvements, update.

[dannon]

Support --limit in gulp's viz plugin building, to isolate particular plugins or sets for rebuilding. Also updates to the (surprisingly) new gulp 5, which drops a slew of older dependencies. WIP in another branch to just de-gulpify all this, but it's not a priority right now; I just wanted to pop this out a set of various other viz fixes I'm working on getting in.

i.e. yarn run gulp pluginsRebuild --limit nora

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[dannon]

So the new gulp is failing on a step that has apparnetly been failing for a while now, just silently. Will update.