Clarify: other axis type values should be strings

[imagejan]

For axis types that are not space, time or channel, the JSON schema only allows string values:

ngff/0.4/schemas/image.schema

Lines 155 to 160 in e1dc738

	"type": {
	"type": "string",
	"not": {
	"enum": ["space", "time", "channel"]
	}
	}

The language in the spec however didn't specify this so far. Let's explicitly specify that other values must be strings.

[github-actions]

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[joshmoore]

Thanks, @imagejan! Because of the current build failures, I've pulled these changes into #242 (and applied to the upcoming 0.5 version)