chore(structs): Add collection interface and array-based implementation for use in core

Fixes #7409
chore(core): provide feedback when autoloader is missing

fixes #7220
feature(users): Username character blacklist can now be altered via plugin hook

Also removes hyphen from the blacklist making it a valid character by default.

Fixes #6189
Merge pull request #7512 from juho-jaakkola/username_blacklist

Username character blacklist can now be altered via plugin hook
Merge pull request #7515 from jdalsem/autoloader-feedback

chore(core): provide feedback when autoloader is missing
feature(webapp): Add support for a basic WebApp Manifest file

Fixes #7493
chore(metastrings): Move metastring table code into a class
Merge pull request #7535 from ewinslow/oo-metastrings

chore(metastrings): Move metastring table code into a class
Merge pull request #7503 from ewinslow/structs-collections

chore(structs): Add collection interface and array-based implementation ...
Merge pull request #7531 from ewinslow/webapp-manifest

feature(webapp): Add support for a basic WebApp Manifest file