feat: Add support for passthrough/unknown CLI options

[haydenbleasel]

Summary

Implements support for passing arbitrary CLI flags through to procedures without explicitly defining each one in the schema. This resolves #161.

Key changes:

• Added allowUnknownOptions?: boolean to TrpcCliMeta interface
• Added hasAdditionalProperties?: boolean to ParsedProcedure interface
• Modified schema parsing to track additionalProperties instead of rejecting them
• Configured Commander.js to allow unknown options when the feature is enabled
• Added parseUnknownOptions() helper to parse and merge unknown CLI options into procedure input
• Maintained backward compatibility: schemas with additionalProperties still fall back to JSON input mode unless allowUnknownOptions is explicitly enabled

Usage

const router = t.router({
  forward: t.procedure
    .meta({ allowUnknownOptions: true })
    .input(z.object({ known: z.string().optional() }).passthrough())
    .query(({ input }) => {
      // input contains both known and unknown options
      console.log(input)
    }),
})

mycli forward --known value --unknown-flag foo --another-flag bar
# input: { known: "value", unknownFlag: "foo", anotherFlag: "bar" }

Features

• Kebab-to-camelCase conversion: --my-option becomes myOption
• Boolean flags: --flag sets flag: true
• Negated flags: --no-flag sets flag: false
• Equals syntax: --key=value works as expected
• Type coercion: Numeric strings become numbers, "true"/"false" become booleans

Requirements

Both conditions must be met for passthrough to work:

1. Procedure meta must have allowUnknownOptions: true
2. Input schema must allow additional properties (e.g. .passthrough() or z.record())

Use Case

This is particularly useful for building CLI wrappers around other tools (like the Ultracite/Biome example mentioned in the issue) where you want to forward flags without manually enumerating every possible option.

Test plan

• Added tests for allowUnknownOptions with passthrough schema
• Added tests for allowUnknownOptions with record schema
• Added tests verifying backward compatibility (schemas with additionalProperties but without allowUnknownOptions fall back to JSON input)
• Added tests verifying that allowUnknownOptions alone (without schema support) doesn't enable passthrough
• All existing tests pass

Documentation

• Added "Passthrough/Unknown Options" section to README
• Updated "Advanced Meta Configuration" to mention allowUnknownOptions
• Added feature to "Features and Limitations" list

Closes #161