feat(builtins): add xo code actions and diagnostics

[mvllow]

So far unable to get this to work properly. Help/feedback is much appreciated. I'll continue playing with this and hopefully get something working here 😌

Closes #702

[jose-elias-alvarez]

These changes seem to make things work on my end:

diff --git a/lua/null-ls/builtins/diagnostics/xo.lua b/lua/null-ls/builtins/diagnostics/xo.lua
index 6e849ea..0903615 100644
--- a/lua/null-ls/builtins/diagnostics/xo.lua
+++ b/lua/null-ls/builtins/diagnostics/xo.lua
@@ -29,9 +29,9 @@ return h.make_builtin({
     filetypes = { "javascript", "javascriptreact", "typescript", "typescriptreact" },
     generator_opts = {
         command = "xo",
-        args = { "--stdin", "--stdin-filename", "$FILENAME" },
+        args = { "--reporter", "json", "--stdin", "--stdin-filename", "$FILENAME" },
         to_stdin = true,
-        -- format = "json_raw",
+        format = "json",
         check_exit_code = function(code)
             return code <= 1
         end,

I think that should work as a starting point. xo seems to be more different from ESLint than I expected, so I think it's a good idea to test this before we merge it. If we find that it's ultimately the same, we could set it up as a variant of eslint, kind of like what we do for eslint_d.

I also see that xo provides rule fixes, so we could investigate whether to add code actions as well, like ESLint. Again, if everything lines up with vanilla eslint, it would be simple to set this up as a variant.

[mvllow]

Thanks for the help 😌 So now we have:

• Diagnostics
• Code actions (using "ESLint" verbiage, may want to change some to say XO?)
• Formatting (this is implementing --fix but on second thought that seems more of a code action thing)

I agree that extending ESLint should be fine. I'm curious why ESLint diagnostics/code actions use format: "json_raw" whereas formatting uses format: "json". Other than that, we could simply overwrite filetypes, command, and args.

[jose-elias-alvarez]

Right, the eslint built-in has simple error handling to show runtime errors as editor diagnostics (like the VS Code extension) so we should also use json_raw here, too.

Formatting is a bit of a special case. We should investigate whether xo supports the --fix-dry-run flag, which is the approach used by the ESLint formatting built-in. Otherwise I think it's best to leave it out, since --fix may cause weird behavior.

[mvllow]

I'll look into the --fix-dry-run and try to extend eslint to reduce duplication. Appreciate the insight 😌

[mvllow]

Removed formatting as --fix is destructive. All tests in make test pass locally. Please let me know if there's anything else that needs tested/completed.

Are docs generated automatically or should I add those? (If not automatic, is there a preference on ordering when adding a new builtin?)

[jose-elias-alvarez]

Docs are still done manually, ideally in alphabetical order.

[jose-elias-alvarez]

Excellent, glad we were able to get this solved with the minimal amount of code. Thanks for the contribution!