[TileMapLayer] Add set_cells to set multiple cells at once

[SpazElectro]

Adds TileMapLayer::set_cells to set multiple cells at once using an array of Vector2i positions, similar to TileMapLayer::set_cells_terrain_connect and TileMapLayer::set_cells_terrain_path.

Why? It's ~3 times faster than setting each tile manually in GDScript.

[Mickeon]

While I do understand the need of this, it would be nice having a formal proposal to gather interest, as well as proving how much faster it is with benchmarks.

[SpazElectro]

Sure thing! Here is a sample benchmark scene, on my machine I get this result: Internal implementation average: 851µs, User implementation average: 2973µs, Improvement: 3.493537x