feat(pack): introduce plugin manifest support

Problem: There is no agreed way for plugins to provide extra information
  about themselves. Like required Neovim version, install/update/delete
  hooks, and dependencies.

Solution: Introduce basic concept of plugin manifest file `pkg.json`
  based on https://packspec.org/.

runtime/lua/vim/pack/health.lua

@@ -200,6 +200,31 @@ local function check_lockfile()
   end
 end
 
+local function check_manifest(manifest, plug_name)
+  local name_str = vim.inspect(plug_name)
+  if vim.tbl_count(manifest) == 0 then
+    health.warn(('Plugin %s has empty or malformed manifest file'):format(name_str))
+    return false
+  end
+
+  local nvim_engine = (manifest.engines or {}).nvim or '*'
+  local ok_version, nvim_version_range = pcall(vim.version.range, nvim_engine)
+  if not ok_version then
+    health.warn(('Plugin %s has malformed `engines.nvim` in manifest file'):format(name_str))
+    return false
+  end
+  --- @cast nvim_version_range vim.VersionRange
+  if not nvim_version_range:has(vim.version()) then
+    health.warn(
+      ('Plugin %s Nvim version requirement %s'):format(name_str, tostring(nvim_version_range))
+        .. (' does not match current version %s'):format(tostring(vim.version()))
+    )
+    return false
+  end
+
+  return true
+end
+
 --- @return boolean Whether a check is successful
 local function check_installed_plugin(plug_name)
   local name_str = vim.inspect(plug_name)
@@ -248,6 +273,11 @@ local function check_installed_plugin(plug_name)
     )
   end
 
+  -- Manifest
+  if info[1].manifest then
+    return check_manifest(info[1].manifest, plug_name)
+  end
+
   return true
 end