`commonmark_x` never actually supported `auto_identifiers` (it
didn't do anything), because the underlying library implements
gfm-style identifiers only.
Attempts to add the `autolink_identifiers` extension to
`commonmark` will now fail with an error.
Closes#6863.
The parameter is Extensions. This allows these functions to
be sensitive to the settings of `Ext_gfm_auto_identifiers` and
`Ext_ascii_identifiers`.
This allows us to use `uniqueIdent` in the CommonMark reader,
replacing some custom code.
It also means that `gfm_auto_identifiers` can now be used
in all formats.
Semantically, `gfm_auto_identifiers` is now a modifier of
`auto_identifiers`; for identifiers to be set, `auto_identifiers`
must be turned on, and then the type of identifier produced
depends on `gfm_auto_identifiers` and `ascii_identifiers` are set.
Closes#5057.
Non-ascii characters were not stripped from identifiers even if the
`ascii_identifiers` extension was enabled (which is is by default for
gfm).
Closes#4742
