Use a resolver for table conversion

[rpaul-stripe]

This PR changes the behavior of the table conversion code action so that it uses a code action resolver instead of a command. A conflict results when more than one instance of the language server registers a command with the same ID, so we can't register a command with a constant name during language server initialization.

In order to use commands for something like this, we'd need to append some unique identifier to the command ID. In this specific case, using a resolver to decouple the edit from the code action provider seemed like a more idiomatic solution.

This PR also updates the version of Markdoc and introduces support for the config.validation.environment variable, defaulting its value to language-server. This allows custom validation functions to identify the context in which they are being executed.