feat(pack): add support for sourcing manifest scripts

Problem: No way for plugins to define hooks that would be executed
  during plugin lifecycle. Like after install/update or before
  update/delete.

Solution: Automatically source scripts defined in plugin manifest file
  (if any) after triggering corresponding `PackChanged{,Pre}` events.
  During sourcing make some termporary adjustments:
  - Current directory is set to plugin's root to simplify execution of
    CLI calls like `make build`.
  - Plugin's path is ensured to be inside 'runtimepath' to allow using
    `require('plugin-module')` inside manifest scripts.

  The reason to execute after triggering event is so that there is a
  possibility for users to execute code both before the script (exactly
  on event) and after the script (more-or-less via `vim.schedule` called
  on the event).

runtime/lua/vim/pack/health.lua

@@ -200,7 +200,8 @@ local function check_lockfile()
   end
 end
 
-local function check_manifest(manifest, plug_name)
+--- @param manifest vim.pack.Manifest
+local function check_manifest(manifest, plug_name, plug_path)
   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))
@@ -222,6 +223,18 @@ local function check_manifest(manifest, plug_name)
     return false
   end
 
+  local ok_scripts = true
+  ---@diagnostic disable-next-line: no-unknown
+  for name, script_path in pairs(manifest.scripts or {}) do
+    if vim.fn.filereadable(vim.fs.joinpath(plug_path, script_path)) == 0 then
+      health.warn(('Plugin %s has no %s script at %s path'):format(name_str, name, script_path))
+      ok_scripts = false
+    end
+  end
+  if not ok_scripts then
+    return false
+  end
+
   return true
 end
 
@@ -275,7 +288,7 @@ local function check_installed_plugin(plug_name)
 
   -- Manifest
   if info[1].manifest then
-    return check_manifest(info[1].manifest, plug_name)
+    return check_manifest(info[1].manifest, plug_name, plug_path)
   end
 
   return true