…(thanks to @alirezac0)

…d mapping events, #784

…ted (#780)

YAML 1.2.2 8.1.3 defines a more-indented ("spaced text") line by production
[175] s-nb-spaced-text, whose leading [33] s-white is a space *or a tab*; the
breaks around such a line are preserved verbatim ([177] b-l-spaced) rather than
folded. getBlockValue() in the parser already tests for both (0x20 or 0x09),
but the presenter tested only for a space, at four sites in foldBlockScalar()
and foldLine(). It therefore doubled breaks the parser keeps literally, adding
one \n per adjacency:

  parseEvents/eventsToAst/present of

    k: >
      <TAB>
      detected

  emitted a blank line before "detected", so the value went from
  "\t\ndetected\n" to "\t\n\ndetected\n". A tab line between two folded lines
  cost two extra \n.

Extract the predicate as isMoreIndented() and use it for the prevMoreIndented
seed, the per-line test and the foldLine guard; the fold-point regexp becomes
/ [^ \t]/ so a break is never placed before a tab either (breaking at the space
in "aaa... \tzzz" moved the tab to the start of the next line).

Enumerating every two-line block-scalar body over a 7-line alphabet in both
block styles gave 84 parseable sources: 12 changed value through
present(), all folded style, all involving a tab; literal style was clean
(0/42). All 12 were confirmed against PyYAML 6.0.3 and ruamel.yaml 0.19.1
reading the emitted bytes, and all 12 round-trip after this change. The public
dump() API never enters this path (it quotes tab-containing strings), so it was
unaffected before and after.