feat(api): add nvim_win_text_height (#24236)

It uses the same code as "scroll_delta" of "win_viewport" UI event to
calculate text height, but is more flexible.

runtime/doc/api.txt

@@ -2961,6 +2961,38 @@ nvim_win_set_width({window}, {width})                   *nvim_win_set_width()*
       • {window}  Window handle, or 0 for current window
       • {width}   Width as a count of columns
 
+nvim_win_text_height({window}, {*opts})               *nvim_win_text_height()*
+    Computes the number of screen lines occupied by a range of text in a given
+    window. Works for off-screen text and takes folds into account.
+
+    Diff filler or virtual lines above a line are counted as a part of that
+    line, unless the line is on "start_row" and "start_vcol" is specified.
+
+    Diff filler or virtual lines below the last buffer line are counted in the
+    result when "end_row" is omitted.
+
+    Line indexing is similar to |nvim_buf_get_text()|.
+
+    Parameters: ~
+      • {window}  Window handle, or 0 for current window.
+      • {opts}    Optional parameters:
+                  • start_row: Starting line index, 0-based inclusive. When
+                    omitted start at the very top.
+                  • end_row: Ending line index, 0-based inclusive. When
+                    omitted end at the very bottom.
+                  • start_vcol: Starting virtual column index on "start_row",
+                    0-based inclusive, rounded down to full screen lines. When
+                    omitted include the whole line.
+                  • end_vcol: Ending virtual column index on "end_row",
+                    0-based exclusive, rounded up to full screen lines. When
+                    omitted include the whole line.
+
+    Return: ~
+        The number of screen lines that the range of text occupy.
+
+    See also: ~
+      • |virtcol()| for text width.
+
 
 ==============================================================================
 Win_Config Functions                                          *api-win_config*

runtime/doc/news.txt

@@ -99,6 +99,9 @@ The following new APIs and features were added.
 
 • |vim.system()| for running system commands.
 
+• Added |nvim_win_text_height()| to get the number of screen lines occupied by
+  a range of text in a given window.
+
 • |nvim_set_keymap()| and |nvim_del_keymap()| now support abbreviations.
 
 • Implemented LSP inlay hints: |vim.lsp.inlay_hint()|