Add public C API for partial "unicode-escape" and "raw-unicode-escape" decoding

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-10-14.18:40:52.507>
labels = ['type-feature', 'expert-unicode', '3.11']
title = 'Add public C API for partial "unicode-escape" and "raw-unicode-escape" decoding'
updated_at = <Date 2021-10-14.19:00:35.314>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2021-10-14.19:00:35.314>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Unicode']
creation = <Date 2021-10-14.18:40:52.507>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 45472
keywords = ['patch']
message_count = 1.0
messages = ['403932']
nosy_count = 4.0
nosy_names = ['lemburg', 'vstinner', 'ezio.melotti', 'serhiy.storchaka']
pr_nums = ['28955']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45472'
versions = ['Python 3.11']