Support "on type formatting" in Zed

[janko]

Motivation

Zed doesn't support snippet syntax ($0) in document/onTypeFormatting responses, nor is it possible for its Ruby extension to implement it. Because the cursor position isn't right after edits, Zed's Ruby extension currently has onTypeFormatting disabled by default for Ruby LSP.

Fixes #2971

Implementation

For "end" and "heredoc end", we can work around this by inserting them in the line below the cursor. This prevents the cursor from moving after the inserted text, avoiding the need to rewind it. We can't support this at the EOF, as there is no next line, but that's the rarer scenario.

This won't work for formatting on the same line, because Zed cannot rewind the cursor. Zed already supports auto-closing brackets, so we skip closing curly braces. We do the same for pipes, because it's better not to add anything than to do it incorrectly.

Automated Tests

I've added unit tests.

Manual Tests

To test this in Zed, you first need to enable on type formatting in Zed settings:

{
  "lsp": {
    "ruby-lsp": {
      "initialization_options": {
        "enabledFeatures": {
          "onTypeFormatting": true
        }
      }
    }
  }
}