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
```
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