fix(*): drop chat re-export from root entry of public API

[damyanpetev]

Fixes #16601

Chat is still accessible through its 'igniteui-angular/chat' entry as intended, just not through the root entry re-exports (that cover backwards compatibility) as it relies on optional dependencies.

Note: Also tried with explicit exports to see it if we can keep in root like

export { IgxChatAttachmentContextDirective, IgxChatComponent, IgxChatInputContextDirective, IgxChatMessageContextDirective } from 'igniteui-angular/chat';

since star exports still need to resolve to figure out the module which could mess some shaking, but nope. Haven't dug too deep, guessing there's a bundle pass that doesn't do shaking first or it doesn't work that way at all.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them