Add a method to get TileData from a cell

[KoBeWi]

So recently I tried the custom data in TileSets and discovered that to get custom data you need to do

var source = tile_set.get_source(get_cell_source_id(0, cell, false))
var data: TileData = source.get_tile_data(get_cell_atlas_coords(0, cell, false), 0)

if data.get_custom_data("actual_data"):
	stuff

That's a lot of writing for something so basic (and this snippet won't even work in all cases). So this PR adds a helper method to get the data easier.

Also it fixes use_proxies not defaulting to false as they should be.

[groud]

I guess that helper makes sense as used often. Maybe yous could add a comment stating this is a helper method, not an accessor vital to TileMap good functioning?

[YuriSizov]

@groud @KoBeWi Can this be moved forward? What needs to be fixed? Also a rebase

[KoBeWi]

Rebased. I removed the use_proxies argument change (moved to #64496).

[YuriSizov]

Thanks!