IDLE: Factor out similar code in editor and hyperparser

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2020-02-17.18:12:00.743>
labels = ['expert-IDLE', 'type-feature', '3.9']
title = 'IDLE: Factor out similar code in editor and hyperparser'
updated_at = <Date 2020-06-13.00:09:42.603>
user = 'https://github.com/csabella'

bugs.python.org fields:

activity = <Date 2020-06-13.00:09:42.603>
actor = 'cheryl.sabella'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2020-02-17.18:12:00.743>
creator = 'cheryl.sabella'
dependencies = []
files = []
hgrepos = []
issue_num = 39666
keywords = ['patch']
message_count = 3.0
messages = ['362153', '371206', '371428']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'taleinat', 'cheryl.sabella']
pr_nums = ['18539', '20847']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39666'
versions = ['Python 3.9']