Turbopack: Move turbopackIgnoreIssue from experimental to turbopack.ignoreIssue (#89817)
## What?
Moves the `turbopackIgnoreIssue` config from `experimental` to the stable `turbopack` config as `turbopack.ignoreIssue`.
## Why?
Reviewers on #89682 agreed the API is stable enough to graduate from experimental. Also addresses a docs wording suggestion.
## How?
- **TypeScript types**: Moved `ignoreIssue` field from `ExperimentalConfig` to `TurbopackOptions` in `config-shared.ts`
- **Zod schema**: Moved validation from `experimentalSchema` to `zTurbopackConfig` in `config-schema.ts`
- **Serialization**: Moved RegExp serialization logic into the turbopack block in `build/swc/index.ts`
- **Rust**: Moved `ignore_issue` field from `ExperimentalConfig` to `TurbopackConfig` in `next_config.rs`, updated the `turbopack_ignore_issue_rules()` method to read from `self.turbopack`
- **Tests**: Updated config in test to use `turbopack.ignoreIssue` instead of `experimental.turbopackIgnoreIssue`
- **Docs**: Removed `version: experimental`, updated all code examples, moved entry from experimental table to stable config list in turbopack.mdx, fixed "Good to know" wording to say "prefer using more specific `path` patterns" (since `path` is already required)