This PR makes the automatic first token detection in tactic docs much
more robust, in addition to making it work in modules and other contexts
where builtin tactics are not in the environment. It also adds the
ability to override the tactic's first token as the user-visible name.
Previously, first token detection would look up the parser descriptor in
the environment and process its syntax. This would be incorrect for
builtin parsers, as well as for modules in which the definition is not
loaded. Now, it instead consults the Pratt parsing table for the
`tactic` syntax category. Tests are added that ensure this keeps working
in modules, and also that the first token of all tactics that ship with
Lean are either detected unambiguously or annotated to remove ambiguity.
Closes#12038.
9fbbe6554d