build: reduce comptime usages in MimeType

[DonIsaac]

What does this PR do?

• Reduce amount of comptime expressions in MimeType, hopefully making builds faster
• Reduce surface area of MimeType's API
• Remove some usages of bun.string

How did you verify your code works?

CI

[robobun]

^{Updated 12:14 PM PT - Mar 31st, 2025}

❌ @Jarred-Sumner, your commit fce2a0d has 2 failures in Build #14299:

• test/js/web/fetch/fetch.test.ts - timeout on 🍎 14 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🪟 2019 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🪟 2019 x64-baseline
• test/js/web/html/FormData.test.ts - 1 failing on 🍎 13 aarch64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 3.21 aarch64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 3.21 x64-baseline
• test/js/web/html/FormData.test.ts - 1 failing on 🍎 14 aarch64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 3.21 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 12 aarch64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 22.04 aarch64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 24.04 aarch64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 20.04 aarch64
• test/js/web/html/FormData.test.ts - 1 failing on 🍎 13 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 22.04 x64-baseline
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 24.04 x64-baseline
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 12 x64-baseline
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 20.04 x64-baseline
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 22.04 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 24.04 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 12 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🐧 20.04 x64
• test/js/web/html/FormData.test.ts - 1 failing on 🍎 14 x64

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🧪   try this PR locally:

bunx bun-pr 18543