Pool commentState allocations

[jakebailey]

These comment states appear to be one of the main allocators when emitting the VS Code codebase. Pool them within a print run.

Reduces the VS Code alloc count from 36015276 to 28140625 (20% reduction).

[Copilot]

Pull Request Overview

This PR introduces a commentStatePool to reuse commentState instances and significantly reduce allocations when printing. It replaces direct allocations of commentState with pooled instances in various emitComments methods.

• Added a core.Pool[commentState] field to Printer
• Updated emitCommentsBeforeNode to retrieve and initialize a pooled commentState
• Switched emitCommentsBeforeToken and emitCommentsAfterToken to use pooled instances instead of fresh allocations

[weswigham]

This seems fine, but Q: Does this need to be a heap allocated pointer at all? printerState, which always hosts it, is by-value already. I'm pretty sure the comment state is created by emitCommentsBeforeNode and then only consumed by emitCommentsAfterNode (or similar token/list functions) at the same print depth, and the fields fit in a few registers easy.

[jakebailey]

emitCommentsAfterNode returns early on nil; I tried using a zero value but it breaks things because a zero value is actually seemingly valid. ☹️