lsp: implement cueimports behaviour under config for Format

[myitcv]

Go has goimports:

goimports updates your Go import lines, adding missing ones and removing unreferenced ones.

Indeed this behaviour is also available via gopls via a config option.

CUE should have something similar.

Prior art here is @asdine's https://github.com/asdine/cueimports.

We should provide a config option and mode for this in cue lsp.

Note: this will require that cache of parsed artefacts, because the local resolution must inspect files in the same package to verify that it is unresolved.