Add support for 'companies' property

**Is your feature request related to a problem? Please describe.**
Feature parity with the Cordova Intercom plugin.

**Describe the solution you'd like**
Add support to update the intercom specific 'companies' property

**Describe alternatives you've considered**
Can use custom attributes instead but Intercom has a special case for the 'companies' property.

**Additional context**
Equivalent code within the intercom cordova plugin is here: https://github.com/intercom/intercom-cordova/blob/master/intercom-plugin/src/ios/IntercomBridge.m#L185

Start by comparing the requested behavior with the equivalent implementation in IntercomBridge.m at the linked line 185. Trace how the Capacitor plugin exposes Intercom property updates on each supported platform, then verify that the special 'companies' property can be updated rather than treated as a custom attribute.

Written by the indexing model from the issue text.