spec: unclear whether # is a valid identifier

[mvdan]

A hash just by itself is not a valid identifier per the spec:

https://cuelang.org/docs/reference/spec/#identifiers

identifier = [ "#" | "_#" ] letter { letter | unicode_digit } .

However, as of 2d28bd3, they clearly work in the current implementation:

exec cue export input.cue

-- input.cue --
#: "foo"
bar: #

> exec cue export input.cue
[stdout]
{
    "bar": "foo"
}

Moreover, they get used by the jsonschema encoding as well:

exec cue def input.json

-- input.json --
{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "$defs": {
        "some-thing": {
            "type": "string"
        }
    }
}

> exec cue def input.json
[stdout]
@jsonschema(schema="https://json-schema.org/draft/2020-12/schema")
_
#: {
	"some-thing": string
}

It may be too late or painful to remove support for hash identifiers at this point, if we never intended them to work. The easier alternative seems to be to adjust the spec to reflect reality. cc @rogpeppe