Add title property to theme.json schema

[t-hamano]

What?

This PR adds the title property to the theme.json schema.

Why?

The title property is not currently defined as a schema. However, it is used as a mouse-over label in global style variations.
When the title property is absent, "Default" is displayed for theme.json and the file name for variations.

How?

Added title property and description to theme.json schema.
If you have a more appropriate description, please advise.

Testing Instructions

Confirm that the code editor outputs an error for the title property when referencing the trunk schema.

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"title": "Variation Name"
}

Confirm that the code editor displays the title property as a candidate and does not output an error when referencing this PR's schema.

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/add/theme-json-schema-title/schemas/json/theme.json",
	"version": 2,
	"title": "Variation Name"
}

cc @mikachan Because we had a talk about the title property in the Twenty Twenty Three PR.

[ajlende]

Looks good to me 👍