add a proc mechanism to Blacklight::Configuration#property to allow deferred resolution of some default values

[barmintor]

• if property default values are not defined under lib, they may not be available until application is loaded, causing issues for downstream apps running generators
• fixes #2705

[barmintor]

I'm not enamored of this mechanism, but... it works. Without something like this, I was left using nil for the default and overriding accessors, which appears to be the 7.x approach we want to move away from.

[barmintor]

Converting this to a draft as there may just need to be zeitwerk changes implemented in the plugins, see https://github.com/projectblacklight/blacklight_range_limit/pull/185

[barmintor]

Closed as was #2705