editorial: export "extension modules" definition (#591)

So that it can be referred from other specs.

Currently there is no definition we can refer to in `webdriver-bidi`:
https://respec.org/xref/?term=extension+modules, unlike WebDriver
classic which has an exported "extension commands" definition.

Bug: #506
Bug: #587

index.bs

@@ -466,10 +466,10 @@ in the <code>ResultData</code> group for the results of commands in the module.
 Modules which contain events define [=local end definition=] fragments that are
 choices in the <code>Event</code> group for the module's [=events=].
 
-An implementation may define <dfn>extension modules</dfn>. These must have a
+An implementation may define <dfn export>extension modules</dfn>. These must have a
 [=module name=] that contains a single colon "<code>:</code>" character. The
 part before the colon is the prefix; this is typically the same for all
-extension modules specific to a given implementation and should be unique for a
+[=extension modules=] specific to a given implementation and should be unique for a
 given implementation. Such modules extend the [=local end definition=] and [=remote end definition=]
 providing additional groups as choices for the defined [=commands=] and [=events=].