Add --typst-input CLI option.

This allows one to pass parameters to typst, which are available
at `sys.inputs`, just as `typst` itself does with its `--input`
option.

[API changes]

* ReaderOptions has a new field `readerTypstInputs`.
* Opt has a new field `optTypstInputs`.

Closes #11588.

test/command/11588.md

@@ -0,0 +1,10 @@
+```
+% pandoc --typst-input foo=FOO --typst-input bar="bar bim" -f typst -t plain
+#sys.inputs.at("foo");
+
+#sys.inputs.at("bar");
+^D
+FOO
+
+bar bim
+```