fix: say when slint init writes a config that shadows an inherited one

[MaximeGaudin]

Summary

• slint init in a subdirectory checked only ./slint.toml, so it happily wrote a config that shadows an inherited parent config for everything linted below — with no sign.
• init now runs the same walk-up lookup the linter uses at run time; when a config already governs the directory, it names that file and says writing here will shadow it. The file is still written (init's job is to write); the warning is the news.
• The current directory is canonicalised before the walk-up, because Path::new(".").parent() is the empty path, which makes the relative walk stop one level short.

Fixes #40

Test plan

• New e2e test failed before the fix (no warning) and passes after
• The test also asserts the no-parent case stays quiet, and the file is still written with exit 0
• Local checks mirror CI and passed before push (./scripts/check.sh --no-js)
• CI green on the PR