Add `Node.startMark`

[tom-tan]

This request is to solve #256 by making dyaml.exception.Mark public and by adding dyaml.node.Node.startMark to get the start position of the node.

Although it lacks some information such as the file name, it is quite useful to provide fine-grained messages.

[codecov[bot]]

Codecov Report

Merging #263 into master will decrease coverage by 0.04%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
- Coverage   94.96%   94.92%   -0.05%     
==========================================
  Files          29       28       -1     
  Lines        4170     4137      -33     
==========================================
- Hits         3960     3927      -33     
  Misses        210      210              

Impacted Files	Coverage Δ
source/dyaml/exception.d	76.47% <0.00%> (-10.20%)	:arrow_down:
source/dyaml/node.d	92.43% <0.00%> (-0.11%)	:arrow_down:
contrib/tinyendian/source/tinyendian.d

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[Herringway]

The user should be able to keep track of the filename (if any) on their own, so that should not be an issue

[tom-tan]

Thanks!