Fix off-by-one in <File> line-range extraction and add regression test

[afurm]

Summary

<File ... start="x" end="y" /> snippet extraction had an off-by-one bug that included one extra line after end.

This PR:

• fixes the line-range slicing so end does not include the next line
• adds a regression test for start="2" end="3" to ensure only lines 2–3 are returned
• adds a small require.main guard/export so the helper can be tested directly without changing normal script execution

Before / After

• Before: start=2, end=3 returned lines 2, 3, 4
• After: start=2, end=3 returns lines 2, 3

Testing

node --test website/scripts/copy-md-to-static.test.js