Support gradients for slide background

[tjinauyeung]

Adds gradient support for slide backgrounds.

Usage

slide.background = { 
  gradient: { 
    angle: 90, 
    stops: { 
      0: "#FF00FF", 
      100: "#00FF00" 
    } 
  } 
}

How to test

1. In your terminal, go to demo folder cd ./demos
2. Run npm i
3. Run node ./browser_server.mjs
4. Open up http://localhost:8000/browser/index.html in the browser
5. Replace the html snippet and run export

[tjinauyeung]

Very imperative code here, but consistent with the style used in the project, see createGlowElement above

[tjinauyeung]

This is handled similar to colors.

When no valid color is found, we log a warning in the console, and use a default color.

See createColorElement fn.

[sam-pitch]

When trying to run the node ./browser_server.mjs I get
I just had to run npm i first.

internal/process/esm_loader.js:74
    internalBinding('errors').triggerUncaughtException(
                              ^

Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'express' imported from /home/sam/Code/repos/PptxGenJS/demos/browser_server.mjs
    at new NodeError (internal/errors.js:322:7)
    at packageResolve (internal/modules/esm/resolve.js:732:9)
    at moduleResolve (internal/modules/esm/resolve.js:773:18)
    at Loader.defaultResolve [as _resolve] (internal/modules/esm/resolve.js:887:11)
    at Loader.resolve (internal/modules/esm/loader.js:89:40)
    at Loader.getModuleJob (internal/modules/esm/loader.js:242:28)
    at ModuleWrap.<anonymous> (internal/modules/esm/module_job.js:76:40)
    at link (internal/modules/esm/module_job.js:75:36) {
  code: 'ERR_MODULE_NOT_FOUND'
}

[sam-pitch]

Looks very good to me!

[zarkone]

looks great!

Interesting that all the compiled code is not in .gitignore

[pastafari]

@tjinauyeung can we make a plan for attempting to merge changes to upstream?
e.g

• Should we make a PR from add-gradients to upstream/master or from pitch-main to upstream/master?
• How do we rebase against changes to upstream?