feat(renderer): interpolate placeholders in bound props

[adamdehaven]

Summary

: and v-bind: string props can now mix fixed text with {{ }} placeholders:

:a[Docs]{:href="/docs/{{ doc.slug }}/overview"}

---
api:
  id: "9b3dc8fc-f3a3-4796-bbbc-400985eafe74"
  slug: "getting-started"
---

::a
---
:href: https://example.com/apis/{{ $doc.api.id }}
---
Link with API ID
::

::a
---
:href: https://example.com/apis/{{ api.slug }}
---
Link with API slug
::

Placeholders resolve against the same data as text bindings. The result is always a string.

Resolves #515

How it works

• New src/runtime/utils/template.ts. MDCRenderer.vue calls it from propsToDataRxBind.
• Lookups use own properties only. Functions never run.
• A linear scanner with limits (4096 characters, 16 placeholders, 256 per placeholder).
• The assembled value is checked with validateProp and the unsafe link prefixes. An unsafe prop is removed with a warning that names the prop only.
• isAnchorLinkAllowed no longer throws on malformed %. Link props treat it as unsafe.

Compatibility

These values keep the old behavior and are not interpolated:

• Props without :, and key, ref, is and style.
• Malformed templates and values over the limits.
• JSON arrays and objects. The parser passes YAML arrays and objects as JSON.

A : prop that already held a well-formed {{ }} now interpolates.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxtjs/mdc@516

commit: a68872d

[adamdehaven]

Also porting forward to Comark: comarkdown/comark#490