feat(stdlib): overload vim.str_byteindex, vim.str_utfindex #30735

PROBLEM:
There are several limitations to vim.str_byteindex, vim.str_utfindex:
1. They throw given out-of-range indexes. An invalid (often user/lsp-provided)
   index doesn't feel exceptional and should be handled by the caller.
   `:help dev-error-patterns` suggests that `retval, errmsg` is the preferred
   way to handle this kind of failure.
2. They cannot accept an encoding. So LSP needs wrapper functions. #25272
3. The current signatures are not extensible.
    * Calling: The function currently uses a fairly opaque boolean value to
      indicate to identify the encoding.
    * Returns: The fact it can throw requires wrapping in pcall.
4. The current name doesn't follow suggestions in `:h dev-naming` and I think
   `get` would be suitable.

SOLUTION:
- Because these are performance-sensitive, don't introduce `opts`.
- Introduce an "overload" that accepts `encoding:string` and
  `strict_indexing:bool` params.

```lua
local col = vim.str_utfindex(line, encoding, [index, [no_out_of_range]])
```

Support the old versions by dispatching on the type of argument 2, and
deprecate that form.

```lua
vim.str_utfindex(line)                             -- (utf-32 length, utf-16 length), deprecated
vim.str_utfindex(line, index)                      -- (utf-32 index, utf-16 index), deprecated
vim.str_utfindex(line, 'utf-16')                   -- utf-16 length
vim.str_utfindex(line, 'utf-16', index)            -- utf-16 index
vim.str_utfindex(line, 'utf-16', math.huge)        -- error: index out of range
vim.str_utfindex(line, 'utf-16', math.huge, false) -- utf-16 length
```

src/nvim/lua/stdlib.c

@@ -181,7 +181,9 @@ int nlua_str_utfindex(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
   } else {
     idx = luaL_checkinteger(lstate, 2);
     if (idx < 0 || idx > (intptr_t)s1_len) {
-      return luaL_error(lstate, "index out of range");
+      lua_pushnil(lstate);
+      lua_pushnil(lstate);
+      return 2;
     }
   }
 
@@ -272,7 +274,8 @@ int nlua_str_byteindex(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
   const char *s1 = luaL_checklstring(lstate, 1, &s1_len);
   intptr_t idx = luaL_checkinteger(lstate, 2);
   if (idx < 0) {
-    return luaL_error(lstate, "index out of range");
+    lua_pushnil(lstate);
+    return 1;
   }
   bool use_utf16 = false;
   if (lua_gettop(lstate) >= 3) {
@@ -281,7 +284,8 @@ int nlua_str_byteindex(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
 
   ssize_t byteidx = mb_utf_index_to_bytes(s1, s1_len, (size_t)idx, use_utf16);
   if (byteidx == -1) {
-    return luaL_error(lstate, "index out of range");
+    lua_pushnil(lstate);
+    return 1;
   }
 
   lua_pushinteger(lstate, (lua_Integer)byteidx);
@@ -695,10 +699,10 @@ void nlua_state_add_stdlib(lua_State *const lstate, bool is_thread)
     lua_setfield(lstate, -2, "stricmp");
     // str_utfindex
     lua_pushcfunction(lstate, &nlua_str_utfindex);
-    lua_setfield(lstate, -2, "str_utfindex");
+    lua_setfield(lstate, -2, "__str_utfindex");
     // str_byteindex
     lua_pushcfunction(lstate, &nlua_str_byteindex);
-    lua_setfield(lstate, -2, "str_byteindex");
+    lua_setfield(lstate, -2, "__str_byteindex");
     // str_utf_pos
     lua_pushcfunction(lstate, &nlua_str_utf_pos);
     lua_setfield(lstate, -2, "str_utf_pos");