RFC: Migrate API constants to enum-like API

The API currently has a huge list of hundreds of capitalized constants that kind of play the part enums (which JS doesn't technically support natively). For example:

https://api.playcanvas.com/engine/variables/ADDRESS_CLAMP_TO_EDGE.html

Let's look at this specific group:

/**
 * Texture data is not stored a specific projection format.
 *
 * @category Graphics
 */
export const TEXTUREPROJECTION_NONE = 'none';

/**
 * Texture data is stored in cubemap projection format.
 *
 * @category Graphics
 */
export const TEXTUREPROJECTION_CUBE = 'cube';

/**
 * Texture data is stored in equirectangular projection format.
 *
 * @category Graphics
 */
export const TEXTUREPROJECTION_EQUIRECT = 'equirect';

/**
 * Texture data is stored in octahedral projection format.
 *
 * @category Graphics
 */
export const TEXTUREPROJECTION_OCTAHEDRAL = 'octahedral';

We could rewrite that to:

/**
 * Texture projection modes for storing texture data. These define how the texture's data is
 * interpreted or sampled, such as cubemap or equirectangular projections.
 *
 * @readonly
 * @enum {string}
 * @category Graphics
 */
export const TextureProjection = Object.freeze({
    /** No specific projection format is used. */
    None: 'none',

    /** Cubemap projection format. */
    Cube: 'cube',

    /** Equirectangular projection format. */
    Equirect: 'equirect',

    /** Octahedral projection format. */
    Octahedral: 'octahedral'
});

So, for properties and functions that took one of those constants and was typed as @type {string}, we could update to @type {TextureProjection}. This would result is a much more nicely structured API reference manual, with related constants grouped on the same page.