A plug-in that went on calling after a failed `vc_call` still had each
later call made on the document, although the run fails and is undone
at its end. After the run's first failed call, `vc_call` now returns
`-11` (CALL_NO_CONTEXT) and the host makes no further call; the run
still fails with the first call's reason.
vc_call returning -11 (from the start function, or in a run that can't make
calls) now fails the run like every other code, so a plug-in that ignores
it can't report success for a call that never ran. Calls are allowed once
the module has loaded, so a start-function call gets -11 instead of
trapping.
A filter at plug-in API level 2 with the new `commands` permission names
the commands it may run in its manifest's `commandAllowlist` and runs
them with `env.vc_call`, reading each result with `env.vc_call_result`.
Each call runs through Session::execute on the run's document and sees
what earlier calls changed; the calls and the filter's output are one
undo step, and a run that fails, is cancelled or makes a failed call
leaves the documents, their history and the session state the commands
change as they were.
Commands that touch files, the clipboard, preferences, the view,
scripts, actions, plug-ins, open documents or the undo history are never
callable, and every other command must be listed as allowed in
cmd/plugin_call/reviewed.rs, so a new command is refused until placed.
Calls are capped at 1,000 a run, 8 MiB a call and 64 MiB a run, and each
costs instructions from the run's budget. plugin.install refuses a
manifest that lists a refused command and reports unknown ids;
plugin.inspect lists both, and the approval dialog lists the commands.
Refs #2186
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