chore: export "WebDriver session" dfn for cross-spec xref resolution (#1959)

The "session" dfn lacked data-export, preventing other specs from
referencing it via [=WebDriver session=] with data-cite="webdriver".

Needed by w3c-fedid/digital-credentials#381 which defines WebDriver
BiDi extension commands that reference the session concept.

Uses data-local-lt="sessions" to preserve existing internal links
without exporting the generic plural term.

Author: marcoscaceres

index.html

@@ -2164,7 +2164,7 @@ <h3>Processing capabilities</h3>
 <section>
 <h2>Sessions</h2>
 
-<p>A WebDriver <dfn data-lt="sessions">session</dfn> represents the
+<p>A WebDriver <dfn data-lt="WebDriver session" data-local-lt="sessions" data-export>session</dfn> represents the
  logical connection between a <a>local end</a> and a
  specific <a>remote end</a>. The <a>session</a> object holds state
  specific to that connection.