Fixed Level Up

## The problem

In vanilla, spending levels (enchanting table, anvil) always costs the same number of *levels*, but a level
gets more expensive the higher you go: 7 points at level 0, 37 at level 15, 112 at level 30, and 9 more for
every level after that. So a 3-level enchant is cheap at level 30 and absurdly expensive at level 300.

## The fix

This mod makes the XP needed to go from one level to the next a fixed amount, so level 30 -> 31 costs the
same as level 100 -> 101. Spending 3 levels on an enchant now always costs the same number of points.

Because this changes how levels are *earned* rather than how they are *spent*, it works with the enchanting
table, anvils, and any other mod that charges levels, without touching any of them.

## Config

`config/fixedlevelup-common.toml`

| Option       | Default | Description                              |
|--------------|---------|------------------------------------------|
| `xpPerLevel` | 45      | XP points every level costs (1 - 100000) |

The default of 45 means reaching level 30 costs 1350 points, close to vanilla's 1395, so the early game
feels about the same while the late game stops getting more expensive. Lower it for faster levelling.

## Notes

- Orb values, mob drops, mending, bottles o' enchanting etc. are untouched; only the level thresholds change.
- Existing worlds keep their level and their *fraction* of progress toward the next level.
- All the XP maths runs on the server and the mod registers no network channels or content, so it should
  only be needed on the server (or in singleplayer). Installing it on clients as well is harmless.
- Mods that compute XP with their own formula instead of asking the game (some XP storage blocks) may
  convert at their own rate.