This commit adds support for return values in partials.
This means that you can now do this and similar:
{{ $v := add . 42 }}
{{ return $v }}
Partials without a `return` statement will be rendered as before.
This works for both `partial` and `partialCached`.
Fixes #5783
This commit adds a "cache potential" column when running `hugo --templateMetrics --templateMetricsHints`.
This is only calculated when `--templateMetricsHints` is set, as these calculations has an negative effect on the other timings.
This gives a value for partials only, and is a number between 0-100 that indicates if `partial` can be replaced with `partialCached`.
100 means that all execution of the same partial resulted in the same output.
You should do some manual research before going "all cache".
