Add support for Runeforged/mastered unique variants

[vaisest]

Description of the problem being solved:

This adds support for runeforged/mastered base variants in the unique DB by adding a new {base:n} variant tag. This works similarly to variants, but is treated as its own group, similar to what has been done with unique versions. This should also work with versioning:

Rarity: Unique
Versioned Base Variant Test
{version:1}{base:1}Gold Ring
{version:1}{base:2}Iron Ring
{version:2}{base:1}Ruby Ring
{version:2}{base:2}Amethyst Ring
Version: Legacy
Version: Current
Selected Version: 1
Base Variant: Gold/Ruby
Base Variant: Iron/Amethyst
Selected Base Variant: 2
Implicits: 0
{version:1}+10 to maximum Life
{version:2}+20 to maximum Life

'Rarity: UNIQUE
Atziri's Step
Cinched Boots
Evasion: 533
League: Fate of the Vaal
Base Variant: Regular Base
Base Variant: Runeforged
Base Variant: Runemastered
Selected Base Variant: 3
{base:1}Cinched Boots
{base:2}Runeforged Cinched Boots
{base:3}Runemastered Cinched Boots
Quality: 20
LevelReq: 65
Implicits: 0
30% increased Movement Speed
{range:0.5}(80-120)% increased Evasion Rating
{range:0.5}+(70-100) to maximum Life
{range:0.5}Gain Deflection Rating equal to (40-60)% of Evasion Rating
{range:0.5}-(12-6)% to amount of Damage Prevented by Deflection
Cannot be Light Stunned by Deflected Hits'

This is still not yet fully implemented. For example the dropdown in the above example doesn't update correctly, and the functionality of adding the runemastered base to a unique which already has base variants needs testing. PoB2 currently seems to have no uniques with base variants.

Currently this defaults to the last base variant, which is runemastered in most cases. I think it's a good idea for sorting purposes, but this needs more work, since lvl > 55 uniques can also be runeforged, but they will convert some defences to ward, instead of being a pure benefit.

Steps taken to verify a working solution:

• Automated tests added

Link to a build that showcases this PR:

Before screenshot:

After screenshot: