Support Rust's `frontmatter` syntax

[epage]

Problem

RFC #3503 adds support for a new frontmatter syntax. The contained content format is defined by the calling tool with a tool identifier.

In RFC #3502, the cargo infostring identifier would be used to signify the block uses the Cargo.toml format/schema. Cargo will assume that an empty infostring is meant to be used with it.

Expected behavior

Support parsing Rust files with frontmatter syntax. Optionally detect and support TOML syntax inside of the frontmatter if the infostring is cargo or if there is no infostring

Note: These are not stable and I'm not expecting this be implemented before then but opened this now as a heads up to prep for when it is. Stabilization is being tracked in:

• Tracking Issue for frontmatter rust-lang/rust#136889
• Tracking Issue for cargo-script RFC 3424 rust-lang/cargo#12207