chore: unretire xmllint

lua/null-ls/builtins/formatting/xmllint.lua

@@ -0,0 +1,20 @@
+local h = require("null-ls.helpers")
+local methods = require("null-ls.methods")
+
+local FORMATTING = methods.internal.FORMATTING
+
+return h.make_builtin({
+    name = "xmllint",
+    meta = {
+        url = "http://xmlsoft.org/xmllint.html",
+        description = "Despite the name, xmllint can be used to format XML files as well as lint them, and that's the mode this builtin is using.",
+    },
+    method = FORMATTING,
+    filetypes = { "xml" },
+    generator_opts = {
+        command = "xmllint",
+        args = { "--format", "-" },
+        to_stdin = true,
+    },
+    factory = h.formatter_factory,
+})