Typst reader: support auto_identifiers extension.

Headings without an explicit label can now be assigned automatic
identifiers based on the heading text, making them linkable in a
generated table of contents.  The extension is available for the
typst reader but is off by default; enable it with
`-f typst+auto_identifiers`.  The related `gfm_auto_identifiers` and
`ascii_identifiers` extensions are also made available.

Closes #11041.

Text.Pandoc.Readers.Typst.Parsing: PState gains sOptions,
sIdentifiers, and sLogMessages fields, and now has
HasReaderOptions, HasIdentifierList, and HasLogMessages
instances, allowing reuse of the shared registerHeader. (Not an
API change.)

Co-Authored-By: Claude <noreply@anthropic.com>

test/command/typst-auto-identifiers.md

@@ -0,0 +1,37 @@
+With `auto_identifiers` enabled, headings without an explicit label get
+automatic identifiers, with collisions disambiguated:
+
+```
+% pandoc -f typst+auto_identifiers -t native
+= Introduction
+
+Text.
+
+== A Sub Section
+
+= Introduction
+^D
+[ Header
+    1 ( "introduction" , [] , [] ) [ Str "Introduction" ]
+, Para [ Str "Text." ]
+, Header
+    2
+    ( "a-sub-section" , [] , [] )
+    [ Str "A" , Space , Str "Sub" , Space , Str "Section" ]
+, Header
+    1 ( "introduction-1" , [] , [] ) [ Str "Introduction" ]
+]
+```
+
+The extension is off by default, so headings have no identifiers:
+
+```
+% pandoc -f typst -t native
+= Introduction
+
+Text.
+^D
+[ Header 1 ( "" , [] , [] ) [ Str "Introduction" ]
+, Para [ Str "Text." ]
+]
+```