Add --allow-fallback option to the map command that controls shifted
and ascii (alternate_key) fallback for individual key mappings.
For non-Latin keyboard layouts, when the current layout key is
non-ascii (codepoint > 127 and < 0xE000), the alternate_key from
the base layout is used for matching if the mapping opts in via
--allow-fallback=shifted,ascii.
Default kitty bindings use --allow-fallback=shifted,ascii so they
work out of the box with non-Latin layouts. User custom mappings
default to --allow-fallback=shifted (preserving existing shifted_key
behavior without ascii fallback).
--allow-fallback=none disables all fallback for a mapping.
Python side: parse_options_for_map() in options/utils.py handles flag
parsing, ShortcutMapping uses it in __init__. get_shortcut() filters
candidates by per-mapping allow_fallback.
Go side: ParseMap() handles --allow-fallback, KeyAction stores
AllowFallback, ShortcutTracker.Match passes it to matching.
MatchesParsedShortcut defaults to shifted,ascii for hardcoded shortcuts.
Migrated kittens (themes, command_palette, diff, choose_files) to
use ShortcutTracker with configurable map entries.
Tests added for Python (5 test methods) and Go (ParseMap + key matching).
