Add support for function keys (F1–F12)

[kirby88]

Summary

Adds support for the function keys F1–F12 as tape commands, so tapes can drive TUIs and apps that bind function keys (e.g. switching tabs/panes).

They follow the exact pattern of existing keypress commands like PageUp, including the optional @<time> and repeat-count grammar:

F1
F3 2
F5@500ms 3

Changes

• token: new F1–F12 token constants, Keywords mappings, and IsCommand entries.
• parser: F1–F12 added to CommandTypes and the parseCommand keypress switch, routing through parseKeypress (so @time/repeat work).
• command: CommandFuncs entries mapping each F-key to ExecuteKey(input.Fn) — go-rod already defines input.F1–input.F12.
• record: F-key terminal escape sequences added to EscapeSequences so vhs record captures them.
• docs: man page list + README "Function Keys" section.
• tests: dedicated lexer/parser F-key tests; updated command_test.go command-count assertions (31 → 43).

No lexer change was required — readIdentifier already accepts digits, so F3 lexes as a single identifier token.

Testing

• go build ./..., go vet ./..., go test ./... all pass; gofmt clean.
• vhs validate accepts a tape using F3, F12@200ms 2, and F1 3.

Notes

• Modifier chords (Ctrl+F3, Shift+F12) are intentionally not included in this PR to keep it focused; happy to add as a follow-up.