Feature/v3/entrypoint schema

[1000TurquoisePogs]

This PR adds a new v3 feature, which appears to be compatible with v2 schema when tested, that you can declare where your plugin entry JS file is for the desktop to load.
This is designed to let you ship a plugin which is compatible with multiple versions of the desktop.

Consider the webcontent section of the sample-angular-app https://github.com/zowe/sample-angular-app/blob/129fc6ca4d06089bb55e0c879782d35c6b751264/pluginDefinition.json#L9

If you add an "entryPoint" section within, then you can state to the Desktop where your entry point file is for each version of Zowe.
v2 is fixed to "web/main.js", but v3 could be variable by this property.
In the example below, the v3 Desktop should load "web/v3assets/main.mjs"

  "webContent": {
    "framework": "angular",
    "launchDefinition": {
      "pluginShortNameKey": "sampleangular",
      "pluginShortNameDefault": "Angular Sample",
      "imageSrc": "assets/icon.png"
    },
    "entryPoint": {
      "2.0": "main.js",
      "3.0": "v3assets/main.mjs"
    },
    "descriptionKey": "sampleangulardescription",
    "descriptionDefault": "Sample App Showcasing Angular Adapter",
    "defaultWindowStyle": {
      "width": 850,
      "height": 540
    }
  },

[DivergentEuropeans]

This PR is difficult to review because the real description appears to be "V2 --> V3 update + schema change"

The smaller portion of the schema bits lgtm so approving

[DivergentEuropeans]

This won't be for 2.0, 3.0 won't be for 3.0 etc. this will be for V2 or V3 right? So let's change it to
"v2" or "2" and "v3" or "3"

[1000TurquoisePogs]

its intended to be a version number, so that if a change is needed there can be a "3.1", "3.2", etc.

[DivergentEuropeans]

This will cause conflicts because updated V2 readme that should be V3 has some of this stuff, but it's ok. Will be simple to merge