feat: add createTableIfNotExists migration helper

[imorland]

Changes proposed in this pull request:
Adds an additional Migration helper - createTableIfNotExists - self explanitory, really.

Reviewers should focus on:
I considered modifying the existing createTable function, with an option 3rd param to specify if this should be conditional, but felt that a seperate, although very similar function, would be clearer in it's intended functionality. Hopefully others will agree?

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[luceos]

I actually think this makes sense.