Allow extensions to opt-out of initialization

[notchairmk]

Problem

The zed_extension_api requires that extensions always return a command (specifically Result<Command>). There are some situations where an extension may be able to discern that it can't or shouldn't initialize, with raising an error being the only way to let the user know that something went wrong. This ends up being somewhat of a hassle to users, potentially requiring them to have custom configs for each workspace just to disable the extension. Ideally the extensions would be able to "just work" from a more global config.

Relevant issue: notchairmk/zed-sorbet#2

Proposed Solution

Allow language_server_command to return a Result<Option<Command>>, conditionally trying to start the extension if it return a command.