feat: add warning-aware import diagnostics

[scarf005]

Summary

• add warning severity plumbing to deno_lint diagnostics and example reporting
• make no-import-prefix and no-unversioned-import warnings with lockfile-aware deno.json(c) autofixes
• avoid guessed versions and handle overlapping fixes when both warnings trigger

Related: denoland/deno#30681

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[bartlomieju]

Verdict: WIP (draft) — needs a rebase onto the merged severity work before it can land. Not merging. Confined mostly to new rules + examples/dlint plus diagnostic plumbing.

Biggest item: the severity plumbing is now duplicated/superseded
This PR re-adds its own LintDiagnosticSeverity { Error, Warning } enum plus diagnostic_level()/highlight_style()/severity() in src/diagnostic.rs. But the target branch already merged LintDiagnostic::severity (#1487): src/diagnostic.rs now defines LintDiagnosticSeverity (Error/Warning) with as_str(), serde round-trip, the severity field defaulting to Error, and the same DiagnosticLevel/highlight-style mapping — all with tests. As-is this PR will hard-conflict and produce duplicate definitions. The plumbing half should be dropped and rebased to build on the existing enum + severity field, keeping only what's genuinely new (the add_warning_with_fixes-style ergonomics if the merged API doesn't already provide them). The wide churn across jsx_*, no_window*, no_process_global, etc. will also need reconciling against the merged changes.

The genuinely new, valuable part: the two import rules

• no-unversioned-import + no-import-prefix as warnings with lockfile-aware deno.json(c) autofixes via fix_with_deno_config_package. The "avoid guessed versions / handle overlapping fixes when both trigger" design is the right instinct — overlapping fixes are exactly where autofixers corrupt code, so good that it's handled.
• Integration tests spin up real temp deno.json/deno.lock projects (unique dirs via pid + AtomicUsize) — solid coverage. Minor: I don't see temp-dir cleanup, so runs may leave deno_lint_no_unversioned_import_* dirs in $TMPDIR; add teardown (or use a scope-guard) to avoid litter.

To finish (draft)

• Rebase onto feat: add LintDiagnostic::severity #1487's severity API (the main blocker).
• New rules → maintainer product decision; add docs/rules/*.md for them.
• Confirm add_warning_with_fixes aligns with whatever the merged per-rule config/severity work (feat: add LintDiagnostic::severity #1487/feat(config): per-rule configuration with severity and options #1534) exposes, rather than a parallel path.

Perf: nothing concerning in the rule paths (per-import checks).

Foundation is reasonable; the work now is mostly reconciling with the severity infrastructure that landed after this was opened.