Readonly helps out with TS's type guards and is also a good best practice. Specifically, readonly strings/numbers have their literal type instead of a generic `string` | `number` type, which can help catch dead code and some common programming mistakes

Part of #78168

Discovered as part of #81574

As part of the fix for #81574, I discovered that a large number of callers were already passing `undefined` for this value. This fix just marks the type as possibly undefined as well

These events should not be mutated by listeners

Saving itself not implemented :)

web - we need to prevent unload when we detect a pending save or auto save because long running operations are not allowed

We seem to use the readonly property way of exposing these values more widely. Standardizing this a bit across the codebase

Fixes #77094

This reverts commit 19d674f4.

Revert "Revert build perf"

This reverts commit ef893f0c.