The line returned but ml_get_buf() may be freed by another call to
ml_get_buf(), so it is necessary to make a copy.

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.

nvim_buf_get_text is the mirror of nvim_buf_set_text. It differs from
nvim_buf_get_lines in that it allows retrieving only portions of lines.

While this can typically be done easily enough by API clients,
implementing this function provides symmetry between the get/set
text/lines APIs, and also provides a nice convenience that saves API
clients the work of having to slice the result of nvim_buf_get_lines
themselves.

In the future we might want to extend the concept of named marks and
adding opts reduces the need of changing the function signature in the
furute.

Adds the following API functions.

- nvim_buf_set_mark(buf, name, line, col)
  * Set marks in a buffer.
- nvim_buf_del_mark(buf, name)
  * Delete a mark that belongs to buffer.
- nvim_del_mark(name)
  * Delete a global mark.
- nvim_get_mark(name)
  * Get a global mark.

Tests:

- Adds test to all the new api functions, and adds more for the existing
  nvim_buf_get_mark.
    * Tests include failure cases.

Documentation:

- Adds documentation for all the new functions, and improves the
  existing fucntion docs.

removing pending virtcol tests

Allow passing in empty array as a shorthand for array with empty string; add more documentation

add check for start_row as well