refactor: isolate root.C and root.JavaScriptCore

[fel1x-developer]

What does this PR do?

Currently,

const C = @import("root").C;

and

const C = bun.C;

are same. This is because src/bun.zig exposes C as a pub const. This should be avoided and only

const C = @import("root").C;

should be accepted.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

[paperclover]

i think all uses of @import("root").C should be moved to bun.C and not the other way around, since bun is the primary namespace in this codebase, not the root file's struct. if anything, the root file should be empty besides pub const bun = ...; and the stuff from main.zig.

[fel1x-developer]

@paperclover Could you give a try for CI check?

[fel1x-developer]

@paperclover , why do we need this?

bun/src/install/windows-shim/bun_shim_impl.zig

Lines 49 to 50 in 4e193b0

	const is_standalone = !@hasDecl(@import("root"), "JavaScriptCore");
	const bun = if (!is_standalone) @import("root").bun else @compileError("cannot use 'bun' in standalone build of bun_shim_impl");

This is causing error since I move JSC to be under Bun namespace (bun.JSC)