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.

test/functional/lua/pos_spec.lua

@@ -14,9 +14,9 @@ describe('vim.pos', function()
     local pos = exec_lua(function()
       return vim.pos(3, 5)
     end)
-    eq(3, pos.row)
-    eq(5, pos.col)
-    eq(nil, pos.buf)
+    eq(3, pos[1])
+    eq(5, pos[2])
+    eq(nil, pos[3])
 
     local buf = exec_lua(function()
       return vim.api.nvim_create_buf(false, true)
@@ -24,9 +24,9 @@ describe('vim.pos', function()
     pos = exec_lua(function()
       return vim.pos(3, 5, { buf = buf })
     end)
-    eq(3, pos.row)
-    eq(5, pos.col)
-    eq(buf, pos.buf)
+    eq(3, pos[1])
+    eq(5, pos[2])
+    eq(buf, pos[3])
   end)
 
   it('comparisons by overloaded operators', function()
@@ -82,9 +82,39 @@ describe('vim.pos', function()
       return vim.pos.lsp(buf, lsp_pos, 'utf-16')
     end)
     eq({
-      buf = buf,
-      row = 0,
-      col = 36,
+      0,
+      36,
+      buf,
     }, pos)
   end)
+
+  it("converts between vim.Pos and extmark on buffer's last line", function()
+    local buf = exec_lua(function()
+      return vim.api.nvim_get_current_buf()
+    end)
+    insert('Some text')
+    local extmark_pos = {
+      exec_lua(function()
+        local pos = vim.pos(1, 0, { buf = buf })
+        return pos:to_extmark()
+      end),
+    }
+    eq({ 0, 9 }, extmark_pos)
+    local pos = exec_lua(function()
+      return vim.pos.extmark(extmark_pos[1], extmark_pos[2], { buf = buf })
+    end)
+    eq({ 0, 9, buf }, pos)
+
+    local extmark_pos2 = {
+      exec_lua(function()
+        local pos2 = vim.pos(0, 9, { buf = buf })
+        return pos2:to_extmark()
+      end),
+    }
+    eq({ 0, 9 }, extmark_pos2)
+    local pos2 = exec_lua(function()
+      return vim.pos.extmark(extmark_pos2[1], extmark_pos2[2], { buf = buf })
+    end)
+    eq({ 0, 9, buf }, pos2)
+  end)
 end)