Use bun.sys more instead of std.fs

[Jarred-Sumner]

What does this PR do?

Use bun.sys more instead of std.fs:

• We cannot use any unreachable when reading files.
• std.fs isn't consistently using the $NOCANCEL variants of libc functions. pread for example is used instead of pread$NOCANCEL.

How did you verify your code works?

CI. This is a somewhat risky PR if we were relying on errors from std

[robobun]

^{Updated 2:15 PM PT - Feb 20th, 2025}

❌ @Jarred-Sumner, your commit 7a02160 has 2 failures in Build #11937:

• test/js/bun/http/serve-body-leak.test.ts - 3 failing on 🐧 3.20 x64-baseline
• test/cli/create/create-jsx.test.ts - 8 failing on 🍎 14 aarch64
• test/cli/create/create-jsx.test.ts - 8 failing on 🍎 13 aarch64
• test/cli/create/create-jsx.test.ts - 8 failing on 🍎 14 x64
• test/cli/create/create-jsx.test.ts - 8 failing on 🍎 13 x64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 22.04 aarch64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 12 aarch64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 20.04 aarch64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 24.04 aarch64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 3.20 aarch64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 22.04 x64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 12 x64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 24.04 x64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 20.04 x64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 3.20 x64
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 12 x64-baseline
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 24.04 x64-baseline
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 20.04 x64-baseline
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 22.04 x64-baseline
• test/cli/create/create-jsx.test.ts - 8 failing on 🐧 3.20 x64-baseline

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🧪   try this PR locally:

bunx bun-pr 17496