Add support for variable-displaying strings in custom inserts

[sgranade]

Right now, custom inserts don’t work with a string that includes a variable to be displayed:

{custom insert: “the value of displayVariable is {displayVariable}”}

That’s prevented because, in render-inserts.ts render() lines 141 and on, the parsing accepts } if it’s inside a string, but any { signals the start of a new insert, even inside a string.

I’m thinking of use cases where I’d like the custom insert to modify text that then will be processed by engine.template.render().

If there’s no negative repercussions, could we let inserts include } inside a string?

[klembot]

Hmm. I'd call it a bug that it doesn't handle { inside of a string argument, so I created an issue to track that here: https://github.com/klembot/chapbook/issues/176

Whether we allow variable interpolation... not sure. I lean more towards giving a custom insert a way to interpolate variables instead of doing it for the insert automatically.