Revisit whether true, null etc can be used as identifiers

[myitcv]

Per discussion with @mpvl

if: true
true: true
null: null

// etc...

https://cuelang.org/play/?id=3aU41xcIysr#cue@export@cue

[mpvl]

For pseudo keywords like if definitely. But for keywords that are also values like true and null, this seems confusing.

[mvdan]

Very much related: cue fmt should probably transform e.g. true: true into "true": true for the sake of clarity, even if we continue allowing true: true to be valid. See https://github.com/cue-lang/cue/issues/2242.