TerminalFont: add bitty support

[Xuepoo]

Bitty (https://github.com/bitty-terminal/bitty) sets TERM_PROGRAM=bitty and implements the kitty font-query protocol, but detectTerminalFontCommon has no bitty branch, so TerminalFont reports Unknown terminal: bitty.

Changes

• Extract the existing kitty DCS query/reply decode into queryKittyFont() (behavior unchanged for kitty).
• Add detectBitty():
  • reuses queryKittyFont() for the ESC P +q kitty-query-font_family;... ST query, which bitty answers directly on the PTY;
  • falls back to bitty +show-font and parses its stable font_family: / font_size: lines when the DCS response is swallowed (multiplexer/PTY wrapper) or on Windows.
• Route processName == "bitty" to detectBitty().

The kitty fallback chain (+kitten query-terminal, then kitty.conf) is unchanged.

Protocol

Request (bitty and kitty):

ESC P +q <hex(font_family)>;<hex(font_size)> ST

Reply (one per key, family first):

ESC P 1+r kitty-query-font_family=<hex(JetBrainsMono Nerd Font)> ST
ESC P 1+r kitty-query-font_size=<hex(12)> ST

Bitty implementation tracking issue: bitty-terminal/bitty#2263.

bitty +show-font output:

font_family: JetBrainsMono Nerd Font
font_size: 12
font_fallback: <optional>

Testing

• cmake -B build -DCMAKE_BUILD_TYPE=Release && cmake --build build -j — builds clean on Linux.
• ./build/fastfetch --structure terminalfont — no regression on the existing detection path.
• Bitty side is covered by exact-bytes parser/runtime tests plus a real-PTY write-once test.

Happy to adjust naming, fallback ordering, or the extraction shape to match project conventions.