feat: extend vim.Pos, vim.Range #36397

Problem:
Using nested `vim.Pos` objects to represent each `vim.Range` object
requires 3 tables for each `vim.Range`, which may be undesirable in
performance critical code. Using key-value tables performs worse than
using array-like tables (lists).

Solution:
Use array-like indices for the internal fields of both `vim.Pos` and
`vim.Range` objects. Use a metatable to allow users to access them like
if they were key-value tables.

---

Problem:
The `vim.Pos` conversion interface for `extmark` indexing does not take
into account the difference in how a position on top of a newline is
represented in `vim.Pos` and `extmark`.
- `vim.Pos`: for a newline at the end of row `n`, `row` takes the value
  `n + 1` and `col` takes the value `0`.
- `extmark`: for a newline at the end of for `n`, `row` takes the value
  `n` and `col` takes the value `#row_text`.

Solution:
Handle this in the `extmark` interface.

---

Problem:
Not all `to_xxx` interfaces have wrapping objects like `to_lsp`.

Solution:
Return unwrapped values in `to_xxx` interfaces where it makes sense.
Accept unwrapped values in "from" interfaces where it makes sense.

---

Problem:
`start` and `end` positions have different semantics, so they can't be
compared. `vim.Range` relies on comparing the `end` and `start` of two
ranges to decide which one is greater, which doesn't work as expected
because this of the different semantics.

For example, for the ranges:

    local a = {
      start = { row = 0, col = 22, },
      end_ = { row = 0, col = 24, },
    }
    local b = {
      start = { row = 0, col = 17, },
      end_ = { row = 0, col = 22, },
    }

in this code:

    local foo, bar = "foo",  "bar"
    --               |---||-|
    --                 b  a

The range `b` is smaller than the range `a`, but the current
implementation compares `b._end` (`col = 22`) and `a.start` (`col = 22`)
and concludes that, since `b.col` is not smaller than `a.col`, `b`
should be greater than `a`.

Solution:
- Use a `to_inclusive_pos` to normalize end positions inside of
  `vim.Range` whenever a comparison between a start and an end position
  is necessary.

runtime/lua/vim/lsp/codelens.lua

@@ -240,7 +240,7 @@ function Provider:on_win(toprow, botrow)
           local range = vim.range.lsp(bufnr, lenses[1].range, client.offset_encoding)
           ---@type [string, string][]
           local virt_text = {
-            { string.rep(' ', range.start.col), 'LspCodeLensSeparator' },
+            { string.rep(' ', range.start_col), 'LspCodeLensSeparator' },
           }
 
           for _, lens in ipairs(lenses) do