Replace the single optional `suggestion` field with a
`Vec<LintSuggestion>`
to allow multiple automatic fix suggestions per lint report

BREAKING CHANGE: The `suggestion` field has been replaced by
`suggestions`

Support file-level lint checks via `check_file` in lint registry
template.

Extend the `unnecessary_render_result_creation` lint to also flag uses
of
`RenderResult::default()` and `RenderResult::from(...)`, and add
`#[derive(Default)]` to `MlintReport` and `MlintLevel` to simplify
struct
instantiation

generation

Introduce `metadata` and `mlint` subcommands, wire up `cargo_metadata`
for manifest analysis, and enable procedural macro support in editors