Use URI Template syntax for URI templates

[handrews]

Currently we're using a PHP-ish ":", but given that hyper-schema is written using RFC 6570 URI Templates we should stay consistent with that.

[handrews]

This will be simpler if json-schema-org/json-schema-spec#228 gets into draft 6, as it will no longer be necessary to inline JSON Pointers into the templates in order to assign a schema for input validation.

[handrews]

Let's do the following:

• Support displaying variables as {foo} rather than :foo as the former syntax not only matches URI Templates, but is also readable in situations where the variable appears somewhere other than as a full path component
• Provide an option for whether to use our nonstandard-but-essential-for-complex-structures JSON Pointer variable syntax or whether to strictly conform with the spec
• Document that we do not support the draft-04 preprocessing rules, because they're extremely complicated and we took them out in draft-06 on the grounds that no one uses them anyway
• Document how much of the URI Template syntax we support (we can add more support as it's requested by actual users)
• At some point I'll file a separate issue for supporting draft-06's "hrefSchema", so let's put this in the draft-04 milestone

[handrews]

This set of packages is being deprecated in favor of the JSON Schema Tools monorepo. The new packages use URI Templates per RFC 6570.

The new UI is still being designed, but the back-end already fills out example curl requests using RFC 6570. Feel free to file at cloudflare/json-schema-tools if you want to track work related to this in the new UI specifically.