Extension scripts (a/k/a "plugins") can now apply any data format
supported by FormatDescriptor to inline data. In particular, it can
now handle variable-length inline strings. The code analyzer
verifies the string structure (e.g. null-terminated strings have
exactly one null byte, at the very end).
Added PluginException to carry an exception back to the plugin code,
for occasions when they're doing something so wrong that we just
want to smack them.
Added test 2022-extension-scripts to exercise the feature.
