Complete D619 PETSCIIKEY implementation

[dansanderson]

• Repair PETSCII lookup tables
• Do not emit events for non-character key presses
• Return $FF on empty queue, so as not to collide with PETSCII $00
• Tweak repeat timing to match ROM 920385's soft repeat

This resolves https://github.com/MEGA65/mega65-core/issues/582 . As of now, this PR does not include the proposed "unified typing event queue" design, which I believe we should do before publicly documenting PETSCIIKEY. I'll keep working on that. If we're comfortable launching kernel fast typing without documenting PETSCIIKEY, then we can include this and the corresponding ROM change in the next release.

Including fast typing as a kernel feature will require setting a hard dependency from a new version of the ROM to a version of the core containing this change. Naturally, the core will work with previous versions of the ROM, but not the other way around. Fast typing in the ROM is requested by https://github.com/MEGA65/mega65-rom-public/issues/7, and is implemented (with the core dependency) on branch https://github.com/MEGA65/mega65-rom/tree/7_keyscan .

[dansanderson]

Per https://github.com/MEGA65/mega65-core/issues/582#issuecomment-1689395981, I will be upgrading this PR to include MODKEY and MODKEYVIEW registers. I may or may not take a crack at a RAWKEY queue, but that's probably worth a separate PR.

[dansanderson]

This PR is ready for review. MODKEY and MODKEYVIEW are functioning.