feat(lsp): render markdown in docs hover #22766

Problem:
LSP docs hover (textDocument/hover) doesn't handle HTML escape seqs in markdown.

Solution:
Convert common HTML escape seqs to a nicer form, to display in the float.
closees #22757

Signed-off-by: Kasama <robertoaall@gmail.com>

test/functional/plugin/lsp/utils_spec.lua

@@ -0,0 +1,75 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+
+describe('vim.lsp.util', function()
+  before_each(helpers.clear)
+
+  describe('stylize_markdown', function()
+    local stylize_markdown = function(content, opts)
+      return exec_lua([[
+        local bufnr = vim.uri_to_bufnr("file:///fake/uri")
+        vim.fn.bufload(bufnr)
+
+        local args = { ... }
+        local content = args[1]
+        local opts = args[2]
+        local stripped_content = vim.lsp.util.stylize_markdown(bufnr, content, opts)
+
+        return stripped_content
+      ]], content, opts)
+    end
+
+    it('code fences', function()
+      local lines = {
+        "```lua",
+        "local hello = 'world'",
+        "```",
+      }
+      local expected = {
+        "local hello = 'world'",
+      }
+      local opts = {}
+      eq(expected, stylize_markdown(lines, opts))
+    end)
+
+    it('adds separator after code block', function()
+      local lines = {
+        "```lua",
+        "local hello = 'world'",
+        "```",
+        "",
+        "something",
+      }
+      local expected = {
+        "local hello = 'world'",
+        "─────────────────────",
+        "something",
+      }
+      local opts = { separator = true }
+      eq(expected, stylize_markdown(lines, opts))
+    end)
+
+    it('replaces supported HTML entities', function()
+      local lines = {
+        "1 < 2",
+        "3 > 2",
+        ""quoted"",
+        "'apos'",
+        "   ",
+        "&",
+      }
+      local expected = {
+        "1 < 2",
+        "3 > 2",
+        '"quoted"',
+        "'apos'",
+        "   ",
+        "&",
+      }
+      local opts = {}
+      eq(expected, stylize_markdown(lines, opts))
+    end)
+  end)
+end)