Support building line blocks with markdown-it hardbreaks

[jdknight]

Provides the ability to build docutils' line-block nodes if an inline element contains hardbreaks. When a hardbreak is detected as a child of an inline token, a line-block will be created and child tokens will be rendered into a prepared line node. Child nodes are placed into a line node until the next hardbreak is detected, where a new line node is created to hold the next child set. The process repeats until all children are processed.

---

This change is in relation to #877. The change should allow any translator type to handle these line breaks, without requiring the raw directive.

With a document:

# Test

PATH        to be explained \
PYTHONPATH  to be explained \
GRUB        to be explained

Example renderings:

(HTML)

(PDF)

(Confluence)

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.14%. Comparing base (d3d7fbb) to head (08f1afa).

Files	Patch %	Lines
myst_parser/mdit_to_docutils/base.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #948   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files          24       24           
  Lines        3418     3441   +23     
=======================================
+ Hits         3081     3102   +21     
- Misses        337      339    +2     

Flag	Coverage Δ
pytests	90.14% <96.29%> (+<0.01%)	⬆️

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