setDefaultIconCollection = (theme, collectionName) => {
    IconCollectionConfiguration.set(theme, collectionName);
  }
Sets the default icon collection for a given theme.

SAP Icons is the default icon collection (that resolves to SAP-icons version 5.x in Horizon theme family and SAP-icons version 4.x in all other themes)
and to display icons from other collections, you have to specify the icon collection in addition to the icon name, for example: "tnt/actor", "business-suite/1x2-grid-layout", etc.
This method allows setting another (built-in or custom) icon collection as default per theme.

<b>Usage</b>
<b>For example</b>, to make "SAP-icons version 5.x" the default icon collection in "sap_fiori_3":

<pre>
setDefaultIconCollection("sap_fiori_3", "SAP-icons-v5");

<ui5-icon name="home"></ui5-icon>
</pre>

<b>For example</b>, to make a custom icon collection (with name "my-custom-collection") the default icon collection in "sap_fiori_3":

<pre>
setDefaultIconCollection("sap_fiori_3", "my-custom-collection");

<ui5-icon name="custom-icon-name"></ui5-icon>
</pre>

@public
@param { string } theme
@param { string } collectionName