Deprecate baseUrl compiler option in TypeScript 6.0

[Copilot]

This PR implements deprecation of the baseUrl compiler option, which will be removed in TypeScript 7.0. The baseUrl option currently performs two functions that can be confusing to users:

1. Acts as a prefix for all entries in paths
2. Acts as a fallback resolution point for bare module imports (often unintended)

Problem

When users configure:

{
  "compilerOptions": {
    "baseUrl": "./src",
    "paths": {
      "@app/*": ["app/*"],
      "@lib/*": ["lib/*"]
    }
  }
}

An import like import * as blah from "blah.js" might resolve to:

• ./src/app/blah.js
• ./src/lib/blah.js
• ./src/blah.js ⚠️ (often undesirable)

The last resolution path is usually unintentional and can cause confusion.

Solution

Starting in TypeScript 6.0, using baseUrl will show a deprecation warning:

Option 'baseUrl' is deprecated and will stop functioning in TypeScript 7.0. 
Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.

Users should migrate to explicit path mappings:

Before:

{
  "compilerOptions": {
    "baseUrl": "./src",
    "paths": {
      "@app/*": ["app/*"],
      "@lib/*": ["lib/*"]
    }
  }
}

After:

{
  "compilerOptions": {
    "paths": {
      "@app/*": ["./src/app/*"],
      "@lib/*": ["./src/lib/*"]
    }
  }
}

To preserve baseUrl fallback behavior (optional):

{
  "compilerOptions": {
    "paths": {
      "*": ["./src/*"],
      "@app/*": ["./src/app/*"],
      "@lib/*": ["./src/lib/*"]
    }
  }
}

Implementation

• Added deprecation check in src/compiler/program.ts using existing deprecation infrastructure
• Supports ignoreDeprecations: "6.0" to silence warnings during transition period
• Added comprehensive test coverage including migration examples
• Updated 70 existing tests to use ignoreDeprecations to prevent test breakage
• All existing functionality preserved - this is a warning-only change

Fixes #62207.

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