Internal: Performance improvements for `getConfig`

`getConfig` is currently a performance bottleneck and should be improved if possible.

Some ideas:

- Short-circuit esbuild generation if "nothing has changed"
- Do "optimistic file-checks" i.e. only check for file existence as part of error handling path to not slow down the critical path (e.g. when checking for existence of `contentlayer.config.ts`)
- Resolve Contentlayer version at compile time instead of dynamically
- Investigate ways to speed up the `import-compiled-contentlayer-config` step