chore(web): spin out all of the predict-helpers.ts file methods into their own source files

Honestly, I should probably spin out all of the predict-helpers.ts file methods into their own source files, with `predict-helpers` as a subdirectory. I've been avoiding pulling that trigger for a while, partly because I'm not sure if we'd want to have it 'land' this release or next.

_Originally posted by @jahorton in https://github.com/keymanapp/keyman/pull/15852#discussion_r3749497700_

Start by reading predict-helpers.ts and inventorying the methods it contains, then determine how the predict-helpers subdirectory should organize them. Done means all methods have been moved into their own source files without changing behavior, with the release timing clarified before starting.

Written by the indexing model from the issue text.