Add constants for the three VRAM tile "blocks"

[Rangi42]

We already have constants for the two VRAM tilemaps:

def TILEMAP0 equ $9800 ; $9800-$9BFF
def TILEMAP1 equ $9C00 ; $9C00-$9FFF

I'd like to also add constants for the three tile "blocks":

def TILEBLOCK0 equ $8000 ; $8000-$87FF
def TILEBLOCK1 equ $8800 ; $8800-$8FFF
def TILEBLOCK2 equ $9000 ; $9000-$97FF

They're significant base values that aren't directly derivable with STARTOF, and even if people would rather use labels, they can anchor VRAM sections like VRAM[TILEBLOCK0], BANK[0].

[DonaldHays]

I'm in favor of this, and exactly as proposed.

Not only do I think they're meaningfully useful in code, but I think it also helps discussions to have standard names to refer to those memory regions. The issue links to the Pan Docs, which already refer to them as tile block 0, block 1, etc. It's convenient to be able to discuss "tile block 1", versus "the tile region starting from $8800", similar to how having conventional names for PPU modes facilitates discussion about them.

[nummacway]

I'm also in favor of this because I suggested this - well, indirectly.

[Rangi42]

Note that v4.3 deprecated the previous VRAM tile-block address constants in c5a09ff. However, those were named _VRAM8000-_VRAM9000, which was more redundant given their values of $8000-$9000. (Also, that was done by one contributor in 2021, well before the more recent revamp of the constants in 2025.)

Edit: the rationale in #18 was indeed that the old constants were "literally just prefixing a few addresses with _VRAM_", and they suggested "replacing them with _VRAM_BG, _VRAM_SHARED, and _VRAM_OBJ [...] would be much more descriptive". At the time @ISSOtm commented that "laying out VRAM in this way is a convention", which is true for those suggested BG/SHARED/OBJ names, but does not apply to the generic TILEBLOCK0-2 names, which are fundamental to the hardware (as controlled by LCDC_BLOCKS).