15540 Make World::flush_commands private (#15553)

# Objective

Fixes #15540 

End-users risk using `World::flush_commands` instead of `World::flush`,
which panics if any queued commands are `spawn`. Hiding
`World::flush_commands` would help avoid calling a potentially panicky
function, and helps alleviate end-user API confusion.

## Solution

This PR updates the function visibility to crate-level, like
`World::flush_entities`, hiding it from the end-user while still making
it accessible for the tests that are currently set up.

## Testing

The change was tested by executing the available tests for `bevy_ecs`.
From what I've gathered, `World::flush_commands` is not used in any
other bevy crate. If further testing is recommended, please inform me!

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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>