fix: declare typescript as an optional peer dependency

[lazerg]

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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: #4132

The CLI plugin imports typescript at runtime, but the package lists it only as a devDependency. Nothing tells the package manager which copy the plugin should get, so it takes whatever happens to be reachable from its install location. In a pnpm workspace with more than one TypeScript around, that can be a different version than the one @nestjs/cli compiles with.

The plugin inspects the compiler's types but compares type.flags against the TypeFlags of its own copy, and TypeScript 6 renumbered them:

	5.9.3	6.0.3
String	4	32
Number	8	64
Enum	32	65536
BigInt	64	128

So with the program on 6.0.3 and the plugin on 5.9.3, a string reads as an enum and a number reads as a bigint:

static _OPENAPI_METADATA_FACTORY() {
    return { name: { required: true, enum: string }, age: { required: true, type: () => BigInt } };
}

The build succeeds and the app throws ReferenceError: string is not defined as soon as the DTO is loaded. The same source with both sides on 6.0.3 emits type: () => String and type: () => Number.

What is the new behavior?

typescript is declared as an optional peer dependency, so the package manager resolves it from the application instead of leaving it to hoisting. In a pnpm workspace the plugin now gets the application's copy linked into its own node_modules:

# before
node_modules/.pnpm/@nestjs+swagger@…/node_modules/@nestjs/swagger
  (no typescript; resolution falls back to the hoisted copy)

# after
node_modules/.pnpm/@nestjs+swagger@…/node_modules/
  typescript -> ../../typescript@6.0.3/node_modules/typescript

The range is ^5.5.0 || ^6.0.0. 5.5 is the floor because the package is ESM: on 5.4 and below import { TypeFlags } from 'typescript' fails with does not provide an export named. Marking it optional follows ts-morph in @nestjs/graphql, which is plugin-only in the same way.

@nestjs/graphql has the same problem and is reported in nestjs/graphql#4139.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Closes #4132.