feat(api): implement nvim_buf_get_text (#15181)

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.

src/nvim/api/private/helpers.c

@@ -411,7 +411,6 @@ void set_option_to(uint64_t channel_id, void *to, int type, String name, Object
   current_sctx = save_current_sctx;
 }
 
-
 buf_T *find_buffer_by_handle(Buffer buffer, Error *err)
 {
   if (buffer == 0) {
@@ -758,6 +757,52 @@ bool buf_collect_lines(buf_T *buf, size_t n, int64_t start, bool replace_nl, Arr
   return true;
 }
 
+/// Returns a substring of a buffer line
+///
+/// @param buf          Buffer handle
+/// @param lnum         Line number (1-based)
+/// @param start_col    Starting byte offset into line (0-based)
+/// @param end_col      Ending byte offset into line (0-based, exclusive)
+/// @param replace_nl   Replace newlines ('\n') with null ('\0')
+/// @param err          Error object
+/// @return The text between start_col and end_col on line lnum of buffer buf
+String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col, bool replace_nl,
+                    Error *err)
+{
+  String rv = STRING_INIT;
+
+  if (lnum >= MAXLNUM) {
+    api_set_error(err, kErrorTypeValidation, "Line index is too high");
+    return rv;
+  }
+
+  const char *bufstr = (char *)ml_get_buf(buf, (linenr_T)lnum, false);
+  size_t line_length = strlen(bufstr);
+
+  start_col = start_col < 0 ? (int64_t)line_length + start_col + 1 : start_col;
+  end_col = end_col < 0 ? (int64_t)line_length + end_col + 1 : end_col;
+
+  if (start_col >= MAXCOL || end_col >= MAXCOL) {
+    api_set_error(err, kErrorTypeValidation, "Column index is too high");
+    return rv;
+  }
+
+  if (start_col > end_col) {
+    api_set_error(err, kErrorTypeValidation, "start_col must be less than end_col");
+    return rv;
+  }
+
+  if ((size_t)start_col >= line_length) {
+    return rv;
+  }
+
+  rv = cstrn_to_string(&bufstr[start_col], (size_t)(end_col - start_col));
+  if (replace_nl) {
+    strchrsub(rv.data, '\n', '\0');
+  }
+
+  return rv;
+}
 
 void api_free_string(String value)
 {