Pad code-span content symmetrically in HtmlToDjot

[dereuromark]

Problem

HtmlToDjot corrupted inline code whose content begins or ends with a backtick. It added the fence-separating space to only the affected end:

<code>`a</code>   ->  `` `a``     ->  <code> `a</code>   (gained a leading space)
<code>a`</code>   ->  ``a` ``     ->  <code>a` </code>   (gained a trailing space)

Djot removes a single leading and trailing space from a code span only when both ends have one and the span is not all spaces. A lone pad space therefore survives as content and changes the value on the next parse.

Fix

When the content needs separating from the fence (it starts or ends with a backtick and not already a space), pad both ends. The strip rule then removes both and restores the exact content:

<code>`a</code>   ->  `` `a ``    ->  <code>`a</code>
<code>a`</code>   ->  `` a` ``    ->  <code>a`</code>

Content that already starts or ends with a real space is untouched (the existing space already separates the fence, so no extra padding is added and the significant space is preserved). All-space content and the symmetric case (`a`) were already correct and stay correct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.80%. Comparing base (5c9e2f0) to head (951618e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #203      +/-   ##
============================================
- Coverage     91.80%   91.80%   -0.01%     
+ Complexity     3452     3450       -2     
============================================
  Files           104      104              
  Lines          9788     9786       -2     
============================================
- Hits           8986     8984       -2     
  Misses          802      802              

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