internal/zstd: replace hardcoded block size with constant

[aimuz]

Replace the hardcoded maximum block size (128<<10) with a named
constant (blockMaximumSize). This improves code readability and
maintainability by using a single source of truth for the maximum
block size value.

Also, introduce constants for magic numbers related to zstd stream
identification and skippable frames, enhancing code clarity.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Aimuz me:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/542895.
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