Add Terrain items (Seeds)

MR Description

• Adds the functionnality of the Grassy, Psychic, Electric and Misty Seeds from Gen7.

Before testing

• Add the items in Studio. Make sure to set their symbol to electric_seed, psychic_seed, misty_seed or grassy_seed.
• Give one to a Pokémon.

Notes to acknowledge before testing

• The code is dynamic and other items or seed can be created through easy moneky-patching.

Tests to realize

• The seed doesn't trigger until a terrain is applied.
• When a terrain is applied but doesn't correspond to the seed, the seed isn't activated.
• When the corresponding terrain is set through any mean, the seed activates and increases a stat of the Pokémon.
• The seed is destroyed afterwards.

Sources related to this MR

• Bulbapedia link
• Pokepedia link
• Discord link of the dev topic