language: make lists open by default

[myitcv]

Structs are open by default, such that:

x: {
    a: 5
}

x: {
    b: 4
}

evaluates to:

x: {
	a: 5
	b: 4
}

The same is currently not true for lists:

y: [3]
y: [_, 4]

evaluates to:

y: incompatible list lengths (1 and 2)

This issue tracks making lists open by default, such that this would not result in an error and instead would result in:

y: [3, 4]

With this change, lists would be closed via the close() builtin, or when referenced via definitions.

See previous discussion at:

• Default to closed structs for {} syntax? #1149 (comment)

[verdverm]

So would I say "a list of 0+ ints"? as nums: [...int]?

What does nums: [int] imply? 0+ ints or 1+ ints?

[myitcv]

Noting down here, that we need to consider how list.Concat could/should behave with respect to arguments that are open/closed, and therefore what the result should be in those situations.