Moves tilde validations to TheTroubleWithTildes

Almost everywhere tildes are checked uses TTwT#validate_tilde now
Re-added help file parsing

It was commented out? What the hell is wrong with me?
Fixes tilde validations on kspawns
Adds validate_tilde to areadata kspawn line
Adds validate_tilde to room edesc keywords
Section has the ability to split self into children

Adds Section methods "split_children" and "has_children?". Individual
section subclasses override has_children? to return true or false
(areadata and areaheader return false).

split_children stores children in self.children, which later get
iterated over to parse them.

section#parse calls split_children to begin, but split_children can
be called separately (like in tests... probably should just rewrite
the tests so that isn't necessary though)
Parsable shifts to @errors instead of self.errors
Implements new child splitting in VnumSection

VnumSection stores children in array rather than hash keyed by vnum.
Also split_* now handled entirely by Section.
Trims dead code