feat: add proselint

This also removes vale from my config. I was annoyed at how much setup
vale needed before it could work, and find that proselint does what I
want without the extras of vale.

lua/plugins/linting.lua

@@ -5,6 +5,7 @@ return {
 			events = { "BufWritePost", "BufReadPost", "InsertLeave" },
 			linters_by_ft = {
 				html = { "djlint" },
+				markdown = { "proselint" },
 				javascript = { "eslint" },
 				typescript = { "eslint" },
 				javascriptreact = { "eslint" },