Adds exstensible dev server module

[vchendrix]

Ability to extend metacatui server.js for a custom metacatui theme

• adds app.js module that is extended by server.js
• Running npm run dev should produce the same results

Tested against 2.21.0
I tested this against the dataone theme from the main@2.21.0 branch using the following configuration.

MetacatUI.AppConfig = {
    theme: "dataone",
    root: "/",
    baseUrl: "https://search.dataone.org",
    d1CNBaseUrl: "https://cn.dataone.org",
}

Executing npm run dev outputs the following informational lines

% npm run dev                             

> metacatui@2.21.0 dev
> node server.js

Setting /components to /Users/val/workspace/metacatui/src/components
Setting /config to /Users/val/workspace/metacatui/src/config
Setting /css to /Users/val/workspace/metacatui/src/css
Setting /font to /Users/val/workspace/metacatui/src/font
Setting /img to /Users/val/workspace/metacatui/src/img
Setting /js to /Users/val/workspace/metacatui/src/js
Now running at http://localhost:3000

Closes #2036

[vchendrix]

@laurenwalker I have been thinking about the name of ./app.js which may be kind of confusing. Should maybe this be renamed to dev.js or something more appropriate? That way it won't get confused with src/js/app.js.

[vchendrix]

@laurenwalker if there is an appropriate place to document how to use this with custom themes, let me know.