concept: on let clauses and declarations

[myitcv]

A concept piece explaining the disintction between let clauses and declarations. Based on cue-lang/cue#3537.

Copying the example from there for ease of reference:

exec cue eval x.cue
-- x.cue --
working1: {
	let x = 5
	if 5 > 3
	let y = 5 {
		p: [x, y]
	}
}

broken2: [
	let x = 5
	if 5 > 3
	let y = 5 {
		p: [x, y]
	}
]

working2: [
	if 5 > 3
	let x = 5
	let y = 5 {
		p: [x, y]
	}
]

[extemporalgenome]

working1 and broken2 seem identical.

[myitcv]

working1 and broken2 seem identical.

broken2 is a list value, working1 is a struct.