fix: correct cell separator detection for magic commands

This commit corrects the cell separator detection that was broken
in the previous commit. The previous commit allowed lines starting
with "# %%" followed by additional comments to be treated as cell
separators. However, this broke the functionality for magic commands
like "# %%timeit", which should not be treated as cell separators.

The fix involves checking whether there is a space after "# %%" before
treating it as a cell separator. This is done by changing the line
separator check in Lua files to `vim.fn.trim(string.sub(line, 1, 5)) == "# %%"`.

Additionally, it was discovered that `line.startswith("# %%")` in Python
files also misidentified magic commands as cell separators when syncing
to Jupyter. Therefore, this has been corrected to `line[:5].strip() == "# %%"`.

This change ensures that magic commands are correctly recognized and
not treated as cell separators.

lua/jupynium/cells.lua

@@ -12,7 +12,7 @@ function M.line_type(line)
     return "empty"
   elseif vim.startswith(line, "# %% [md]") or vim.startswith(line, "# %% [markdown]") then
     return "cell separator: markdown"
-  elseif vim.fn.trim(line) == "# %%" then
+  elseif vim.fn.trim(string.sub(line, 1, 5)) == "# %%" then
     return "cell separator: code"
   elseif vim.startswith(line, "# ---") then
     return "metadata"

src/jupynium/buffer.py

@@ -75,7 +75,7 @@ def full_analyse_buf(self, header_cell_type: str = "header"):
                 num_rows_per_cell.append(num_rows_this_cell)
                 num_rows_this_cell = 1
                 cell_types.append("markdown")
-            elif line.startswith("# %%"):
+            elif line[:5].strip() == "# %%":
                 num_rows_per_cell.append(num_rows_this_cell)
                 num_rows_this_cell = 1
                 cell_types.append("code")