Add support to create schema for variables

[beandrad]

Given a map of module variables, create the corresponding body
schema with the corresponding variables as attribute schemas.

Related to hashicorp/terraform-ls#50.

[beandrad]

waiting for support for sensitive attributes (hashicorp/hcl-lang#42)

[radeksimko]

LGTM aside from the extra blank lines. 👍🏻

I would probably prefer for this to be merged and deal with IsSensitive in a separate following PR because it will require some other changes here as well so that provider-flagged sensitivity is reflected too:

terraform-schema/schema/convert_json.go

Lines 104 to 111 in 85a7417

	cAttrs[name] = &schema.AttributeSchema{
	Description: markupContent(attr.Description, attr.DescriptionKind),
	IsDeprecated: attr.Deprecated,
	IsComputed: attr.Computed,
	IsOptional: attr.Optional,
	IsRequired: attr.Required,
	Expr: exprConstraintsFromAttribute(attr),
	}

[radeksimko]

btw. I would usually recommend an additional test which runs Validate(), but I reckon that would be failing at this point as per our conversation in hashicorp/hcl-lang#42 (comment) so we can probably just add it later when earlydecoder gains the ability to parse default values.

This will make it an interesting test for hcl-lang decoder to see how it deals with schema that was previously seen as "invalid" (not optional nor required) 😅