feat: add --split-exp-no-overwrite flag to refuse overwriting existing files

[toller892]

What

Adds an opt-in --split-exp-no-overwrite flag that makes --split-exp refuse to overwrite any pre-existing file at a target path, instead failing fast with a clear error message. The existing default behaviour (silent overwrite via os.Create) is unchanged.

Closes #2028.

Why

When yq splits documents into per-file outputs, file names are usually derived from the input data itself, e.g.:

yq -s '.metadata.name + "_" + .kind' all.yaml

If two input documents map to the same name, or if a target path happens to already exist on disk, the second write silently clobbers whatever was there because os.Create truncates. That's a footgun for CI pipelines and bulk-conversion scripts where the user would much rather see an error and fix the expression than discover a missing/overwritten file later.

The issue author asked for an explicit opt-in flag rather than changing the default, which is what this PR does.

How

• Added a new yqlib constructor NewMultiPrinterWriterWithOptions(expression, format, noOverwrite) that carries the new option.

• The original NewMultiPrinterWriter is preserved and now just delegates to the new constructor with noOverwrite=false, so the public API is fully backwards compatible.

• When noOverwrite is true, the writer uses os.OpenFile(name, O_WRONLY|O_CREATE|O_EXCL, 0600). If the target exists, yq returns:

  Error: refusing to overwrite existing file "file_0.yml" (--no-overwrite is set)
  

  and leaves the existing file's contents untouched.

• Wired up a new --split-exp-no-overwrite persistent flag on the root command, defaulting to false.

• Updated the --help snippet in README.md to include the new flag.

Tests

Added pkg/yqlib/printer_writer_test.go covering three scenarios:

1. Default behaviour — existing files are still silently truncated/overwritten (regression guard).
2. --no-overwrite refuses existing files — GetWriter returns an error containing "refusing to overwrite", and the pre-existing file's contents are verified to be untouched.
3. --no-overwrite still creates new files — confirms the flag doesn't break the normal "first write" path.

All existing tests pass:

$ go test ./...
ok  	github.com/mikefarah/yq/v4	1.385s
ok  	github.com/mikefarah/yq/v4/cmd	0.620s
ok  	github.com/mikefarah/yq/v4/pkg/yqlib	1.049s
ok  	github.com/mikefarah/yq/v4/test	1.806s

Manual smoke test

$ printf 'a: 1\n---\nb: 2\n' | yq -s '"file_" + $index'
$ ls
file_0.yml  file_1.yml

# Default: silent overwrite (existing behaviour)
$ printf 'X: 9\n---\nY: 8\n' | yq -s '"file_" + $index'
$ cat file_0.yml
X: 9

# With --split-exp-no-overwrite: fails fast, leaves file alone
$ printf 'A: 7\n---\nB: 6\n' | yq -s '"file_" + $index' --split-exp-no-overwrite
Error: refusing to overwrite existing file "file_0.yml" (--no-overwrite is set)
$ cat file_0.yml
X: 9    # untouched