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
```

runtime/lua/vim/_meta/builtin.lua

@@ -112,18 +112,6 @@ function vim.rpcrequest(channel, method, ...) end
 --- equal, {a} is greater than {b} or {a} is lesser than {b}, respectively.
 function vim.stricmp(a, b) end
 
---- Convert UTF-32 or UTF-16 {index} to byte index. If {use_utf16} is not
---- supplied, it defaults to false (use UTF-32). Returns the byte index.
----
---- Invalid UTF-8 and NUL is treated like in |vim.str_utfindex()|.
---- An {index} in the middle of a UTF-16 sequence is rounded upwards to
---- the end of that sequence.
---- @param str string
---- @param index integer
---- @param use_utf16? boolean
---- @return integer
-function vim.str_byteindex(str, index, use_utf16) end
-
 --- Gets a list of the starting byte positions of each UTF-8 codepoint in the given string.
 ---
 --- Embedded NUL bytes are treated as terminating the string.
@@ -173,19 +161,6 @@ function vim.str_utf_start(str, index) end
 --- @return integer
 function vim.str_utf_end(str, index) end
 
---- Convert byte index to UTF-32 and UTF-16 indices. If {index} is not
---- supplied, the length of the string is used. All indices are zero-based.
----
---- Embedded NUL bytes are treated as terminating the string. Invalid UTF-8
---- bytes, and embedded surrogates are counted as one code point each. An
---- {index} in the middle of a UTF-8 sequence is rounded upwards to the end of
---- that sequence.
---- @param str string
---- @param index? integer
---- @return integer # UTF-32 index
---- @return integer # UTF-16 index
-function vim.str_utfindex(str, index) end
-
 --- The result is a String, which is the text {str} converted from
 --- encoding {from} to encoding {to}. When the conversion fails `nil` is
 --- returned.  When some characters could not be converted they