Add missing JSON field to docs generator output for constants

[straight-shoota]

Fixes #6202

The id field was missing in the JSON serialization of constants, resulting in an undefined value when using it to generate link targets for search results.
I added Constant#id and used this in the template as well, instead of reusing Constant#name to be more explicit (although both methods return the same value) and to have a consistent interface like the other classes where id and name are usually different.

[bcardiff]

@straight-shoota Could you point where the id read and missing?

[straight-shoota]

crystal/src/compiler/crystal/tools/doc/html/js/_search.js

Lines 128 to 139 in ee71f88

	results.push({
	id: constant.id,
	type: type.full_name,
	result_type: "constant",
	name: constant.name,
	full_name: type.full_name + "#" + constant.name,
	value: constant.value,
	summary: constant.summary,
	href: type.path + "#" + constant.id,
	matched_fields: matchedFields,
	matched_terms: matches
	});

[bcardiff]

Great! thanks for clarifying.