fix(command): support Shift/Alt + arrow and navigation keys

[nityanand123gupta]

Fixes #641.

Note: there was a previous attempt at this in #672, which went unreviewed and was auto-closed for staleness back in March. I read that PR for context before writing this one, but the fix here is my own independent implementation against current main, restructured to share logic between ExecuteShift/ExecuteAlt rather than duplicating the keyword switch, and it includes parser-level regression tests that #672 didn't have.

Root cause

Two separate gaps combine to make Shift+Down (and friends) a no-op:

1. Parser: parseShift/parseAlt only accept a bare STRING, or the ENTER/TAB keyword tokens, as the argument after Shift+/Alt+. Since Down, Up, PageUp, Home, etc. are all registered keywords (not plain strings), Shift+Down fails to parse at all — this is exactly the "compile error" the reporter hit.
2. Executor: even if parsing were fixed, ExecuteShift/ExecuteAlt's keyword switch only has cases for token.ENTER and token.TAB — every other keyword would silently fall through and do nothing.

Fix

• Added a modifierKeywordArgs set in parser/parser.go covering every keyword vhs already knows how to type standalone (arrows, PageUp/PageDown, Home/End, Backspace/Delete/Insert, Space, Escape, ScrollUp/ScrollDown), and use it in both parseShift and parseAlt.
• Added the matching modifierKeywordKeys map in command.go and a shared typeModifiedKey helper, replacing the near-duplicate bodies of ExecuteShift/ExecuteAlt. This also makes an unrecognized keyword an explicit error instead of a silent no-op, since the parser and executor keyword sets are now the same source of truth in spirit.

Testing

• Added TestParseShift and TestParseAlt in parser/parser_test.go, covering every newly-supported keyword plus the existing Enter/Tab/literal-character cases and the no-argument error case.
• Verified via git stash on parser/parser.go alone that these new subtests fail without the fix (Shift+Up/Down/Left/Right/PageUp/PageDown/End/Backspace etc. all failed to parse) and pass with it restored.
• go build ./... and go test ./... pass; gofmt -l and go vet ./... are clean on the changed files.