After talking it over with Claude Code, it turns out we can squeeze out a
bit more performance from `prepend_to_selector()` by getting `str_replace()`
to do most of the work.

For one site I was using to test with I found that there were over 4,000
calls to `prepend_to_selector()` just to bootstrap WordPress.  Each one
runs very quickly, but with that volume it would be nice to make it a bit
faster.

For this test with over 4,000 calls, here are the before and after numbers:

Before:
- p75: 0.008032 ms

After:
- p75: 0.006198 ms

While those raw times are going to vary depending on the site and server,
the relative time is what I would focus on.  That 0.001834 ms improvement
is 22.8%.

I had curl make 100 requests and had it log the time via:

```php
$start = hrtime( true );
$elements = static::get_block_element_selectors( $root_selector );
error_log( ( hrtime( true ) - $start ) / 1e6 );
```

Then sorted the logged results with:

```bash
tail -n 100 /tmp/php-errors | awk '{print $NF}' | sort -n | awk 'NR==50{p50=$1} NR==75{p75=$1} END{print "p50:",p50,"\np75:",p75}'
```

I also did a comparison run to make sure that all 4,000+ results were the
same before and after this change.  To `protect prepend_to_selector()` in
the future I also had Claude Code generate tests.

There is also a completely stand alone benchmark script to compare both of
these at https://gist.github.com/josephscott/254521a43dee3300448ef288dd2ea1f2

While not my original focus for this change, it does have a small reduction
in the amount of memory used.

Before:
- p75: 17,874,280 bytes

After:
- p75: 17,870,680 bytes

The 3,600 bytes is veryminor, but nice to see.

Another thing that I considered was getting rid of the separate method
call entirely, since it is only called in one place - inside the
`get_block_element_selectors()` loop.  Now that it is only 4 lines long there
is not much point to calling out to separate method.  Again, trying to
squeeze everything we can out of something called 4,000+ times.  But this
method has been around for a few years, so I didn't worry about that.

See WordPress#50266 for the original PR
that created `prepend_to_selector()`.