presenter: parse notes when reading slide document #2
Description
From https://pkg.go.dev/golang.org/x/tools/present:
[...] Lines that begin with ": " are treated as presenter notes, in both Markdown and legacy present syntax. By default, presenter notes are collected but ignored.
[...]
Notes may appear anywhere within the slide text. [...]
With a new block parser these notes could be parsed easily and then sorted as needed when processing the document.