perf(packages/codegen): ensure indent strings are flattened (#26107)

Codegen pre-prepares strings for writing indentation and caches them.

This PR ensure that these strings are properly flattened. They already were - `indent.charCodeAt(0)` does that. But that's only the case as long as `growIndents` doesn't get tiered up to TurboFan. If it does, TurboFan treats `indent.charCodeAt(0)` as a no-op, and removes it as dead code, which removes the flattening.

`growIndents` is a cold path, so it's unlikely to get tiered up, but make sure that even if it does, we don't lose the flattening by using `indent = [indent, indentString].join("");` instead.

No effect on benchmarks, because TurboFan wasn't kicking in here.