fix: inconsistent YAML multiline parsing (#475)

[guilgaly]

When the YAML block scalar (multiline string) syntax is used with only one line of text, the single line is not matched by the current regexp. It currently only works with at least two lines.

This fixes the regexp by adding a special case for a single line after the first indentation in the block.

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This is a fix for issue #475.

I tested the regexp used in the lexer, it simply doesn't match the content of the block scalar if it contains only one line; I guess the rendering inconsistency came from that line then being matched as something else.

The regexp uses a backreference to check that following lines (after the first) all have at least as much indentation as the first line; but this doesn't work when there is only the first line. I basically just added a special case when there is only one line.

[guilgaly]

This is my first contribution here, please let me know if I should add anything else! 🙂

[alecthomas]

Lovely, thanks!