fix(lua): not obvious which _meta/ files are generated #39035

Problem:
- Not obvious which _meta/ are generated and which should be edited
  manually.
- The require guard (`error('Cannot require a meta file')`) is not
  consistently present in all meta files.

Solution:
- Update headers.
- Add require() guard to all meta files.
- Rename generated meta files with `.gen.lua`.

runtime/lua/vim/_meta/api.gen.lua

@@ -1,6 +1,7 @@
 --- @meta _
--- THIS FILE IS GENERATED
--- DO NOT EDIT
+--
+-- DO NOT EDIT. GENERATED BY: src/gen/gen_eval_files.lua
+--
 error('Cannot require a meta file')
 
 --- This file embeds vimdoc as the function descriptions

runtime/lua/vim/_meta/api_keysets.gen.lua

@@ -1,6 +1,7 @@
 --- @meta _
--- THIS FILE IS GENERATED
--- DO NOT EDIT
+--
+-- DO NOT EDIT. GENERATED BY: src/gen/gen_eval_files.lua
+--
 error('Cannot require a meta file')
 
 --- @class vim.api.keyset.buf_attach

runtime/lua/vim/_meta/api_keysets_extra.lua

@@ -1,4 +1,5 @@
 --- @meta _
+-- This file is NOT generated, edit it directly. See also _meta/api_keysets.gen.lua.
 error('Cannot require a meta file')
 
 --- Extra types we can't generate keysets for

runtime/lua/vim/_meta/base64.lua

@@ -1,4 +1,6 @@
 --- @meta
+-- This file is NOT generated, edit it directly.
+error('Cannot require a meta file')
 
 vim.base64 = {}
 

runtime/lua/vim/_meta/builtin.lua

@@ -1,4 +1,5 @@
 ---@meta
+-- This file is NOT generated, edit it directly.
 -- luacheck: no unused args
 
 error('Cannot require a meta file')

runtime/lua/vim/_meta/builtin_types.lua

@@ -1,3 +1,7 @@
+--- @meta
+-- This file is NOT generated, edit it directly.
+error('Cannot require a meta file')
+
 --- @class vim.fn.sign
 --- @field group string
 --- @field id integer

runtime/lua/vim/_meta/json.lua

@@ -1,4 +1,6 @@
 ---@meta
+-- This file is NOT generated, edit it directly.
+error('Cannot require a meta file')
 
 ---@nodoc
 vim.json = {}

runtime/lua/vim/_meta/lpeg.lua

@@ -1,4 +1,5 @@
 --- @meta
+-- This file is NOT generated, edit it directly.
 error('Cannot require a meta file')
 
 -- These types were taken from https://github.com/LuaCATS/lpeg

runtime/lua/vim/_meta/misc.lua

@@ -1,4 +1,6 @@
 ---@meta
+-- This file is NOT generated, edit it directly.
+error('Cannot require a meta file')
 
 -- luacheck: no unused args
 

runtime/lua/vim/_meta/mpack.lua

@@ -1,4 +1,6 @@
 --- @meta
+-- This file is NOT generated, edit it directly.
+error('Cannot require a meta file')
 
 -- luacheck: no unused args
 

runtime/lua/vim/_meta/options.gen.lua

@@ -1,6 +1,7 @@
 --- @meta _
--- THIS FILE IS GENERATED
--- DO NOT EDIT
+--
+-- DO NOT EDIT. GENERATED BY: src/gen/gen_eval_files.lua
+--
 error('Cannot require a meta file')
 
 ---@class vim.bo

runtime/lua/vim/_meta/re.lua

@@ -1,4 +1,5 @@
 --- @meta
+-- This file is NOT generated, edit it directly.
 error('Cannot require a meta file')
 
 -- Documentations and Lua types for vim.re (vendored re.lua, lpeg-1.1.0)

runtime/lua/vim/_meta/regex.lua

@@ -1,4 +1,6 @@
 --- @meta
+-- This file is NOT generated, edit it directly.
+error('Cannot require a meta file')
 
 -- luacheck: no unused args
 

runtime/lua/vim/_meta/spell.lua

@@ -1,4 +1,6 @@
 --- @meta
+-- This file is NOT generated, edit it directly.
+error('Cannot require a meta file')
 
 -- luacheck: no unused args
 

runtime/lua/vim/_meta/vimfn.gen.lua

@@ -1,6 +1,7 @@
 --- @meta _
--- THIS FILE IS GENERATED
--- DO NOT EDIT
+--
+-- DO NOT EDIT. GENERATED BY: src/gen/gen_eval_files.lua
+--
 error('Cannot require a meta file')
 
 --- Return the absolute value of {expr}.  When {expr} evaluates to

runtime/lua/vim/_meta/vvars.gen.lua

@@ -1,6 +1,7 @@
 --- @meta _
--- THIS FILE IS GENERATED
--- DO NOT EDIT
+--
+-- DO NOT EDIT. GENERATED BY: src/gen/gen_eval_files.lua
+--
 error('Cannot require a meta file')
 
 --- @class vim.v

runtime/lua/vim/_meta/vvars_extra.lua

@@ -1,4 +1,5 @@
 --- @meta _
+-- This file is NOT generated, edit it directly. See also _meta/vvars.gen.lua.
 error('Cannot require a meta file')
 
 --- Extra types for vim.v dictionary fields