update for zig 0.15.2

[nbaertsch]

• replaced usingnamespace and @cImport with TranslateC in the build system (move @cImport to the build system ziglang/zig#20630, Remove usingnamespace ziglang/zig#20663)
• added jsonStringifyAlloc() to handle the new buffered writer interface in Zig 0.15

[nbaertsch]

Turns out std.json.Stringify.valueAlloc() exists so wrapping a writer is unnecessary. Removed the helper, this should be g2g now assuming the github actions passes.