Allow tonconnect/ui to be created twice

[chrisands]

Currently every time TonConnectUI initialized it tries to add to customElements and when application changes pages and initialized second time it throws an error

Uncaught DOMException: Failed to execute 'define' on 'CustomElementRegistry': this name has already been used with this registry

I added condition to check if customElements exists.

Not sure, but probably should fix #171
Tested in Solidjs app.

[chrisands]

@thekiba rebased on main branch. Should be okay now.