Better support for `@embed` in CUE files

[jansorg]

https://cuelang.org/docs/howto/embed-files-in-cue-evaluation/

0.12 enables @embed by default.

It would be possible to provide smarter support for @embed.

For this line

oneFile:   _ @embed(file=README.md,type=text)

The plugin could

• add a link on README.md to the actual file on disk. The user could click it to navigate or use "go to declaratoin" to do the same
• provide autocompletion at file= to show files
• provide autocompletion for the available types after type=

It's probably possible to provide all this via LSP integration.