parser/md: add buttons

[x1ddos]

The button parsing was removed in #33.
The button node creation already exists in

tools/claat/parser/md/parse.go

Line 693 in d557ecb

btn := types.NewButtonNode(true, true, dl, nodes...)

but because isButton always returns false, the node is never created.

For context, the button was here:

tools/claat/parser/md/parse.go

Line 443 in 48ed6f8

ps.emit(types.NewButtonNode(true, true, true, newBreaklessTextNode(s[1])))

/cc @samtstern @didrocks