lang: add `INIT_SPACE_INCL_DISC`

[cryptopapi997]

INIT_SPACE is very useful, but I'd wager in 80%+ of cases what you really want is INIT_SPACE + 8 to include the discrimator. This PR adds INIT_SPACE_INCL_DISC, which is exactly that. Saves a bit of typing & removes the need to remember "magic numbers" (at least for devs who might not be as familiar with what the discriminator exactly is).

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[cryptopapi997]

Might also make sense to export the DISCRIMINATOR_SIZE const from this PR, just ran across https://solana.stackexchange.com/questions/9522/is-there-an-inbuilt-constant-for-anchor-discriminator-size. Wdyt @acheroncrypto ?

[acheroncrypto]

Thanks for the PR, but dynamic discriminators are coming soonish, please see https://github.com/coral-xyz/anchor/issues/3005#issuecomment-2149285238.

[cryptopapi997]

Didn't see this, my bad!