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refactor(argparse): drop the program name from argv with a list pattern

`default_argv` returned everything after `@env.args()[0]` via a length
guard plus an `args[1:]` slice. The `> 1` bound only exists because
`args[1:]` would panic on an empty array, so the emptiness handling is
spread across a comparison and a branch. Express the intent directly:
`[_, .. rest] => rest` drops the head element, with `[] => []` as the
total empty-array fallback. `.. rest` binds an `ArrayView[String]` — the
same zero-copy view `args[1:]` produced — so behaviour is unchanged.

A structural sweep for sibling sites (moongrep over `match` blocks that
destructure the head off a sequence and return the tail with an empty
fallback, plus `rg` over length-guarded `[1:]` slices) found no further
instances. The remaining `[.. rest]`-shaped code in the tree — char/byte
decoders peeling one element per iteration, `FixedArray::rev` and
`debug::compact_lines` needing the last element, quickcheck shrink and
`ArrayView::join` reconstruction — genuinely uses the head or tail, so a
slice or drop is not equivalent there.

## Verification
- `moon check argparse` clean
- `moon fmt --check argparse` clean
- `moon test argparse`: 153 passed, 0 failed

Co-Authored-By: SeekMoon <seekmoon@moonbitlang.com>

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