Add MastodonMetadata highlight group

closes #30

* Refactors the Mastodon plugin to use a new `MastodonMetadata` highlight group
  for rendering metadata in the buffer.
* Previously, the plugin was using the `Whitespace` highlight group,
  which was not appropriate for this purpose.

The new `MastodonMetadata` highlight group is defined with the same foreground
 and background colors as the `NormalFloat` highlight group, with a blend value
 of 10 to provide semi-transparent color. This ensures that the metadata
 could not seen as bloated

These changes improve the readability of Mastodon Buffer

lua/mastodon.lua

@@ -14,11 +14,18 @@ M.setup = function(args)
   M.config = vim.tbl_deep_extend("force", M.config, args or {})
 
   namespace = vim.api.nvim_create_namespace("MastodonNS")
+  local hl_for_whitespace = vim.api.nvim_get_hl(0, { name = "NormalFloat" })
+
   vim.api.nvim_set_hl(namespace, "MastodonHandle", {
     fg = "#000000",
     bg = "#ffffff",
     underline = true,
   })
+  vim.api.nvim_set_hl(namespace, "MastodonMetadata", {
+    fg = hl_for_whitespace.bg,
+    bg = hl_for_whitespace.bg,
+    blend = 10,
+  })
 end
 
 M.toot_message = function()

lua/mastodon/renderer.lua

@@ -272,7 +272,8 @@ local function render_statuses(bufnr, win, statuses, buf_name, mode)
 
   for _, metadata_for_line in ipairs(metadata) do
     vim.api.nvim_buf_set_extmark(bufnr, mastodon_ns, offset + metadata_for_line.line_number, 0, {
-      virt_text = { { metadata_for_line.data, "Whitespace" } },
+      virt_text = { { metadata_for_line.data, "MastodonMetadata" } },
+      hl_mode = "blend",
     })
   end