Use length caching in JS `for` snippet

The previous version of this snippet referenced the object's length on
each iteration of the loop. This adds unnecessary overhead, causing the
loop to be slower. While this will not make much of a difference in many
cases, if the for loop was to be used on a large object or in a tight
loop, it would degrade performance.

Perhaps more importantly, if you were to not cache the length of a live
query, such as DOM queries, you would see significant performance
degradation. If nothing else, this adjustment to the snippet sets things
up for success in more scenarios.
Increase scope of JS `for` snippet placeholder

The `Things.length` part of the JS `for` snippet previously did not
include the `.length` portion. Using some regex replacement, we can
include this bit in the placeholder while still only using the `Things`
bit inside the `for` loop. This should make the snippet slightly more
pleasant to use.
Improve descriptions of JS `for` snippets

More specifically than "faster" and not faster, these snippets count
down and count up, respectively. Adding this information to the
descriptions will help people make the correct decision.