Update dev_setup.sh to use and benefit from bash double square bracket syntax
Update dev_setup.sh to use and benefit from bash double square bracket syntax

+ Consequently use double square brackets (over single square backets), to be consistent and benefit from this syntax
+ Skip obsolete quoting of left sided variables and strings in double square brackets
+ Use "-z" instead of comparing with empty string explicitly in double square brackets
+ Merge multiple double square bracket expressions, connected by "&&" or "||", into a single expression
+ "! -z" => "-n"
+ Remove obsolete free spaces from command substitutions, to be consistent
forslund/mycroft-core,Dark5ide/mycroft-core,MycroftAI/mycroft-core,Dark5ide/mycroft-core,forslund/mycroft-core,MycroftAI/mycroft-core