Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

AI-assisted

Enable checks for unused code, deprecated calls, return types, and
annotations.

Fix the warnings and replace the deprecated highlight function. Keep
exceptions where old APIs are still needed or the checker gets it wrong.

AI-assisted

Problem:
`get_lines()` may returns empty table when file opening fails,
so every existing caller use `get_line() or ''` to avoid nil result.

This also does not match the annotated return type of `get_line()`,
which is `string` instead of `string?`.

Solution:
Make `get_line()` return empty string when file opening fails.

Problem:
`nvim_buf_get_lines` will always returns a table,
so the `or` operator will never be used, letting `lines[row]` may be `nil`

Solution:
Fix it.

Problem:
`get_lines()` actually supports passing a `integer` instead of `integer[]`,
but it is never used in this way, we use `get_line()` instead.

Solution:
Fix it. Also rename some variables to align with our current naming convention
and use `vim.fn.readblob()` instead  of a bunch of `uv` calls.

Problem:
- To share logic, creating a `vim.Range` currently creates two `vim.Pos` values
  as intermediates, which causes unnecessary table allocations.
- `pos.lua` and `range.lua` contain some overlapping logic.

Solution:
Add `vim.pos._util`, a module for handling
positions represented directly by `row` and `col`.