feat: add Bun runtime support (bun test, bun run, bunx)

Context

Bun is a fast JavaScript runtime used as the default in many modern projects. RTK has no support for it — no Rust module, no TOML filter, no rewrite rule.

RTK already supports Node.js ecosystem tools (npm, pnpm, vitest, tsc, eslint, prettier, next, playwright, prisma) but the Bun equivalents are missing.

Scope

Command	Description	Expected savings
rtk bun test	Bun's built-in test runner (similar to vitest)	85-90%
rtk bun run <script>	Script runner (similar to npm run)	70-80%
rtk bun install	Package install (similar to pnpm install)	80-90%
rtk bunx <pkg>	Package executor (similar to npx)	70-80%
rtk bun build	Bundler output	75-85%

Rewrite rules

The hook should rewrite:

• bun test → rtk bun test
• bun run <script> → rtk bun run <script>
• bun install → rtk bun install
• bunx <pkg> → rtk bunx <pkg>
• bun build → rtk bun build

Implementation

Options:

1. Rust module (src/bun_cmd.rs) with sub-enum — best for bun test (structured output parsing)
2. TOML filters for simpler commands (bun install, bun build)
3. Hybrid — Rust for test, TOML for the rest

References

• Vitest module (similar test runner): src/vitest_cmd.rs
• pnpm module (similar package manager): src/pnpm_cmd.rs
• npm module: src/npm_cmd.rs