Allow setting vendor/ location from an ENV variable

[netcarver]

Defaults to existing vendor/ location, but allows this to be overridden by reading the PW_VENDOR_PATH environment variable.

For example, setting PW_VENDOR_PATH to ../vendor/ allows composer to install code outside the site root.

See

• https://processwire.dev/integrate-composer-with-processwire/

[BernhardBaumrock]

@netcarver is on fire :) Thx for all your contributions!!