fix: Avoid re-folding; use existing folds

This commit changes the way that the folding function works. Instead of
always creating a new fold (which is what the function previously did),
it now looks to see if the cursor is already in an open fold, and if so,
it will simply close the fold. If there is no fold, one will be created
following the usual logic.

lua/mkdnflow/folds.lua

@@ -78,29 +78,35 @@ end
 
 M.foldSection = function()
     local row, line = vim.api.nvim_win_get_cursor(0)[1], vim.api.nvim_get_current_line()
-    local in_fenced_code_block = utils.cursorInCodeBlock(row)
-    if M.getHeadingLevel(line) < 99 and not in_fenced_code_block then
-        local range = get_section_range()
-        if range then
-            vim.cmd(tostring(range[1]) .. ',' .. tostring(range[2]) .. 'fold')
-        end
-    else
-        local start_row = get_nearest_heading()
-        if start_row then
-            local range = get_section_range(start_row)
+    -- See if the cursor is in an open fold. If so, and if it is not also on a heading, close the
+    -- open fold.
+    if vim.fn.foldlevel(row) > 0 and not (M.getHeadingLevel(line) < 99) then
+        vim.cmd.foldclose()
+    else -- Otherwise, create a fold
+        local in_fenced_code_block = utils.cursorInCodeBlock(row)
+        -- See if the cursor is on a heading
+        if M.getHeadingLevel(line) < 99 and not in_fenced_code_block then
+            local range = get_section_range()
             if range then
                 vim.cmd(tostring(range[1]) .. ',' .. tostring(range[2]) .. 'fold')
             end
+        else -- The cursor isn't on a heading, so find what the range of the fold should be
+            local start_row = get_nearest_heading()
+            if start_row then
+                local range = get_section_range(start_row)
+                if range then
+                    vim.cmd(tostring(range[1]) .. ',' .. tostring(range[2]) .. 'fold')
+                end
+            end
         end
     end
 end
 
 M.unfoldSection = function(row)
     row = row or vim.api.nvim_win_get_cursor(0)[1]
-    local foldstart = vim.fn.foldclosed(tostring(row))
-    if foldstart > -1 then
-        local foldend = vim.fn.foldclosedend(tostring(row))
-        vim.cmd(tostring(foldstart) .. ',' .. tostring(foldend) .. 'foldopen')
+    -- If the cursor is on a closed fold, open the fold.
+    if vim.fn.foldlevel(row) > 0 then
+        vim.cmd.foldopen()
     end
 end