Use indicator line for props on block collection values of mappings

[eemeli]

Fixes #399

When stringifying a block collection inside a block mapping, the results look nicer if any anchor or tag on the inner collection are placed on the same line as the : indicator.

To effect this, it turns out to be necessary to do a bit of re-parsing of the stringified inner value to determine if it has a leading "props line" or not. In almost all cases, this is limited to looking at the first character of the output, so the performance implications should be negligible esp. as it was already necessary to find the position of the first \n in the output.

The changes made here also alter the indentation of leading comments of block sequence values to match the - indicator column, rather than the value column.