Revert "mkdir errors on already-created dir" - #19008

The default mkdir stays -p-like/idempotent (making it error by default was
reverted in nushell#19008 as a breaking change). Per the maintainer's suggestion on
already exists ("<path>: File exists", matching coreutils) while still
creating non-existing targets in the same call. Default behavior unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

…#19021)

# Description

Follow-up to #19003. The default `mkdir` behavior (idempotent,
`-p`-like) stays exactly as-is — #19006 tried making it error by default
and was reverted in #19008, since people rely on `mkdir foo` to just
ensure a directory exists (and `^mkdir -p` doesn't error either). Per
@fdncred's suggestion in the thread, this instead adds an **opt-in**
flag.

With `--fail-if-exists`, `mkdir` errors on a target that already exists
(message `<path>: File exists`, matching coreutils), while still
creating any non-existing targets passed in the same call. Without the
flag, nothing changes.

The `--verbose` `created: true` half of the issue was already fixed in
#19017, so this covers the remaining piece.

# User-Facing Changes

New opt-in switch: `mkdir --fail-if-exists <dir>` (short: none) errors
if the directory already exists instead of silently succeeding. Default
`mkdir` is unchanged.

# Tests + Formatting

Added `commands::umkdir` tests: errors on an existing dir with the flag,
still creates new dirs with the flag, and `-v --fail-if-exists` reports
`created: false`. Existing default-behavior tests (idempotent mkdir,
intermediary dirs) still pass. `cargo fmt --all --check`, `cargo clippy
-p nu-command`, and the `nu-command` test suite are clean.

# After Submitting

Nothing needed (no doc/`std` changes).

---
_Disclosure: I use AI assistance for code review and for drafting text
(English isn't my first language); I review and verify every change
myself before sending it._

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Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>