Add Tetris Only practice mode

[Knuckleykong]

Add Tetris Only mode

New practice mode where:

• Tetrises score normally.
• Singles, doubles, and triples subtract their normal point value.
• Score is clamped at zero.

Implementation

• Added MODE_TETRISONLY.
• Added a menu entry for Tetris Only.
• Added Tetris Only scoring logic.
• Refactored line-clear point calculation into a shared helper.

Menu

Adding an additional game mode required a menu alignment adjustment so menu selections continue to correspond to the correct mode after reducing the number of blank rows at the top of the menu.

[Knuckleykong]

Note: On my local machine, build.js reports a Node.js
"invalid table size / heap out of memory" error after the
link step, although the ROM is successfully generated and
functions correctly. I have not yet determined whether this
is environment-specific.