fix: improve string width calculation

[teatimeguest]

I found that sometimes notifications are not rendered correctly. Here is an example:

local notify = require("notify")
local title = "\u{1D4AF}\u{212F}\u{1D4C8}\u{1D4C9}" -- "𝒯ℯ𝓈𝓉"

notify("Title not correctly highlighted", nil, { title = title, render = "compact" })
notify("Borderline too short", nil, { title = title, render = "simple" })

Such glitches can be caused by several factors: UTF-16 surrogate pairs (accidentally taken into account here and here), combining character sequences, fullwidth characters, etc.

To fix the problem, this PR makes the following changes:

• Use Lua's # operator to determine the position of extmarks, as it requires simply byte indices.
• Use [vim.api.nvim_strwidth](https://neovim.io/doc/user/api.html#nvim_strwidth()) instead of vim.fn.strchars or vim.str_utfindex to more reliably calculate the width of a string on screen.

Tested with NeoVim v0.5.1 and v0.10.0-nightly.

Limitations

vim.api.nvim_strwidth works well in many cases, but is not perfect (e.g., for emojis).