Make `IChatRequestVariableEntry` more OOP like

[jrieken]

Follow-up from https://github.com/microsoft/vscode/pull/249469

The IChatRequestVariableEntry type (and all its variants) has grown into a shape that needs cleanup. Today, it's open data and util-function. We should use OOP instead, give them some common shape and work on better encapsulation.

For instance,

• there should be toJSON and fromJSON logic so that IPC and persistence is covered
• concrete subtypes should narrow down the type of value
• we should revisit all places that consume value