[graphiql] Copy headers from previous tab when opening a new tab

It's a pain to have to manually copy the (usually always the same) headers from existing tabs into a newly opened tabs. It's slow and annoying.

Pls add an option in the settings to enable a "automatically copy headers from existing tabs" functionality.

Start by tracing GraphiQL’s settings flow and the entry point used when opening a new tab, then identify how existing tab headers are stored and applied. Done means a user-configurable setting copies headers into newly opened tabs while preserving the current behavior when disabled.

Written by the indexing model from the issue text.