fix(api)!: correctly handle negative line numbers for nvim_buf_set_text (#17498)

nvim_buf_set_text does not handle negative row numbers correctly: for
example,

    nvim_buf_set_text(0, -2, 0, -1, 20, {"Hello", "world"})

should replace the 2nd to last line in the buffer with "Hello" and the
first 20 characters of the last line with "world". Instead, it reports
"start_row out of bounds". This happens because when negative line
numbers are used, they are incremented by one additional number to make
the non-negative line numbers end-exclusive. However, the line numbers
for nvim_buf_set_text should be end-inclusive.

In #15181 we handled this for nvim_buf_get_text by adding a new
parameter to `normalize_index`. We can solve the problem with
nvim_buf_set_text by simply availing ourselves of this new argument.

This is a breaking change, but makes the semantics of negative line
numbers much clearer and more obvious (as well as matching
nvim_buf_get_text).

BREAKING CHANGE: Existing usages of nvim_buf_set_text that use negative
line numbers will be off-by-one.

test/functional/api/buffer_spec.lua

@@ -423,6 +423,13 @@ describe('api/buf', function()
       -- will join multiple lines if needed
       set_text(0, 6, 3, 4, {'bar'})
       eq({'hello bar'}, get_lines(0,  1, true))
+
+      -- can use negative line numbers
+      set_text(-2, 0, -2, 5, {'goodbye'})
+      eq({'goodbye bar', ''}, get_lines(0, -1, true))
+
+      set_text(-1, 0, -1, 0, {'text'})
+      eq({'goodbye bar', 'text'}, get_lines(0, 2, true))
     end)
 
     it('works with undo', function()