Debug variables: better support for collections

[kubukoz]

Is your feature request related to a problem? Please describe.

When you debug a program and look into a collection, you have to dig into its specific fields (which are implementation-dependent) to look at a particular element of the collection. It's not very convenient and (in some situations) may be outright impossible because of the arbitrary ordering of the elements' representation in the (deeply nested) fields.

Describe the solution you'd like

Something similar to what IntelliJ does: showing collection elements as fields. If there's a large number of them, you get a preview of a couple elements and can expand the list on demand.

Describe alternatives you've considered

Manually digging into the fields.

Additional context

IDEA's view:

Lazy expansion:

View in VS Code using Metals:

Maps in IDEA:

Maps in VS Code using Metals:

Search terms

debug, dap, debug adapter protocol, collections, stdlib

[adpi2]

That's an interesting feature.

In terms of implementation we could invoke toArray on the variable if its type extends Seq.