feat!: upgrade to initializers system in Bridgetown 1.2

[andrewmcodes]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [x] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [x] Yes
• [ ] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] All tests are passing
• [x] New/updated tests are included

Other information:

I modeled these changes after those done in

[rickychilcott]

Hey @andrewmcodes looks like you might be able to wrap this up now that 1.2 it out.

[andrewmcodes]

@rickychilcott yep just don't have the bandwidth atm and it's not a major priority since you can still do it both ways but hoping to get back to this in the nearish future