x/tools/cmd/present2md: support full translation

[johanbrandhorst]

Not sure this should be considered part of #33955. Please close if so.

The new present2md tool converts the present format to markdown. However, it is still missing some pieces of the present format when converting, namely:

• Captions
• Images

There may be more types I've missed, but these were the ones I noticed so far.

These are being caught by the fallback PresentCmd interface here:

https://github.com/golang/tools/blob/11a475a590acad44f466dde7d3e988cf52bad01c/cmd/present2md/main.go#L236-L247

I think it should be possible to convert both of these to markdown, though the caption may be more complicated.

[cagedmantis]

/cc @ianthehat @matloob

[matloob]

/cc @rsc @bcmills @jayconrod