Markdown target: escape a < that would open markup

[dereuromark]

Found while surveying recent sibling-project fixes for things that apply here. Mirrors markup-carve/carve-php#1241.

MarkdownRenderer::escapeText() escapes the Markdown metacharacters (\ ` * _ [ ] #) and leaves < alone. So djot text is emitted as Markdown that a CommonMark reader parses as raw HTML.

On master, this djot:

a<b>c

renders to this Markdown:

a<b>c

and every CommonMark reader takes <b> as an HTML tag. The authored text is gone and markup appears in its place.

The rule

A < is escaped with a backslash when the next character is an ASCII letter, /, ! or ? - the four things that open raw HTML. Every other < is left alone, and > takes nothing: inert mid-line, and at line start it is a block quote marker the line-level handling already covers.

A backslash rather than an entity. The job is to protect the character so it reads back as itself; an entity replaces it with something else.

djot text	Markdown before	Markdown after
a<b>c	a<b>c	a\<b>c
a < b and 3 > 2	unchanged	unchanged
x<!DOCTYPE y	x<!DOCTYPE y	x\<!DOCTYPE y

One line, appended after the metacharacter pass so the backslash it inserts is not escaped a second time.

Scope

escapeText() only, which serves Text nodes and a div's title. Code spans, code blocks, autolinks, raw blocks and raw inlines never reach it - a test pins that a code span holding <b> keeps its content verbatim, and that a <https://example.com> autolink still renders as a Markdown link.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.42%. Comparing base (72442e7) to head (ef78026).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #273   +/-   ##
=========================================
  Coverage     92.42%   92.42%           
  Complexity     3682     3682           
=========================================
  Files           109      109           
  Lines         10440    10441    +1     
=========================================
+ Hits           9649     9650    +1     
  Misses          791      791           

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