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fix: keep literal backslash before space in link destination (CommonMark 6.3) (#1188)

A backslash followed by a space inside a link destination is a literal
backslash (a space is not ASCII punctuation, so it is not escaped), and
the space ends the destination. markdown-it instead broke at the
backslash, dropping it from the destination and failing to parse the
link.

Repro:

    md.render('[a](/url\\ )')

Expected (matches the CommonMark 0.31.2 reference renderer):

    <p><a href="/url%5C">a</a></p>

Actual on master:

    <p>[a](/url\ )</p>

In lib/helpers/parse_link_destination.mjs the unenclosed-destination
loop, on seeing `\` followed by a space, called `break`, which excludes
the backslash from the destination and leaves `\ )` unconsumed so the
link fails. Advance past the backslash instead (`pos++; continue`) so it
is kept literal and the following space triggers the normal terminator.

The existing commonmark/CommonMark#493 case `[a](a\ b)` is unaffected:
content after the space still prevents the link from parsing, matching
the reference.

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