fix: stats ngram window size cli parsing (#2946)

Fix panic when running "atuin stats" with an empty window size:

    $ RUST_BACKTRACE=1 atuin stats year -n0

thread 'main' panicked at crates/atuin-history/src/stats.rs:278:14:
      window size must be non-zero
      stack backtrace:
         0: __rustc::rust_begin_unwind
         1: core::panicking::panic_fmt
         2: core::option::expect_failed
         3: atuin_history::stats::compute
         4: atuin::command::client::Cmd::run_inner::{{closure}}
         5: tokio::runtime::context::scoped::Scoped<T>::set
6: tokio::runtime::scheduler::current_thread::CoreGuard::block_on
         7: tokio::runtime::context::runtime::enter_runtime
         8: atuin::command::AtuinCmd::run
         9: atuin::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a
      verbose backtrace.

The ngram window size had a default value of 1 (indicating that perhaps
this case was considered), but did not validate the parsed value when a
non-default value was given.

Such validation can either occur:

1. during clap argument parsing
2. after clap parses arguments, but before computing statistics

If we were to do this after clap finishes parsing arguments, this would
be a runtime check in run(), but it seems better to return an error as
soon as possible.

It's possible to do this without a custom parsing function via
clap::value_parser! and specifying a range:

    clap::value_parser!(u64).range(1..)

However, this does not appear to support custom error messages so we
would be left with the default message specifying an invalid range.

Therefore, add a custom parser function and return an error when the
value cannot be parsed as a valid ngram value.

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## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

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