Implement BKM as literal expression

[barmac]

What should we do?

• I can morph BKM to literal expression
• I can open BKM
• I can write BKM body as literal expression
• I can specify parameters of BKM

Why should we do it?

Part of https://github.com/camunda/product-hub/issues/1858

Related to #704

[barmac]

@lmbateman We will need to discuss how we want the user to specify the parameters.

[barmac]

Today, @nikku and I talked about this topic.

We concluded that it makes sense to have a new project within dmn-js family dedicated for all boxed expressions with nesting in mind. Initially, this will support BKM as literal expression. In the future, we can implement other boxed expressions as well as migrate the existing ones (decision table, literal expression for decision).

There's also an idea to upgrade the rendering engine to migrate off Inferno 5, but it's a separate story which can be implemented after the BKM as literal expression is done.

[barmac]

Current state of this is available at https://boxed-expression.netlify.app/

There's still much to do regarding style, but at least the core features work.

[barmac]

@lmbateman and I discussed today the UI part and agreed on the following:

1. The formal parameters will be displayed more like in the specification first, and open the modeling view once edit button is clicked ("pencil").
2. The expression will be displayed without box.