App::env() + parseEnv() improvements

[brandonkelly]

Description

• craft\helpers\App::env() now supports parsing nested environment variables, e.g. FOO="bar/${BAZ}".
• craft\helpers\App::parseEnv() now supports parsing nested environment variables using the …/$NAME/… and …${NAME}… syntaxes anywhere within the passed-in string (not just beginning with $NAME/…).
• CRAFT_SITE and CRAFT_SITE_UPPER environment variables are now defined when the current site is determined.
  • CRAFT_SITE is set to the site’s handle.
  • CRAFT_SITE_UPPER is set to the site’s handle, converted to UPPER_SNAKE_CASE.

So for example, it’s now possible to define per-site API token environment variables, such as:

API_KEY_EN=xxxxxx
API_KEY_DE=yyyyyy
API_KEY_NL=zzzzzz

…and then reference each of them dynamically, depending on the current site, by setting an API token setting to:

$API_KEY_${CRAFT_SITE_UPPER}`.

Related issues

• FR: Site specific email settings #8709