Port lib replacement and --libReplacement

[jakebailey]

Ports lib replacement itself along with microsoft/TypeScript#60829.

The default in this PR is libReplacement=false, unlike Strada, per the deprecation plan. But, easy to flip back if we think it's a problem.

[Copilot]

Pull Request Overview

This PR ports the library replacement behavior and introduces the new --libReplacement flag, updating baseline outputs and error messages accordingly. Key changes include updates to baseline diffs for type outputs and error messages, removal of redundant library processing code in compiler program initialization, and new caching logic for resolving library file paths.

Reviewed Changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated no comments.

File	Description
testdata/baselines/reference/submodule/compiler/*.diff	Updated baseline outputs to reflect the new lib replacement behavior
internal/compiler/program.go	Removed explicit lib processing block in favor of simplified processedFiles call
internal/compiler/parsetask.go	Removed redundant subtask addition to prevent use of an undefined variable
internal/compiler/fileloader.go	Added caching for library file paths and new helper functions for library name resolution

Comments suppressed due to low confidence (2)

internal/compiler/parsetask.go:80

• The removed subtask addition was using the variable 'name' outside the conditional block, which could lead to undefined behavior. The current fix appears correct; please ensure that its removal is intentional and well-tested.

			if name, ok := tsoptions.GetLibFileName(lib.FileName); ok {

internal/compiler/program.go:175

• [nitpick] The removal of the explicit library processing block in the Program constructor simplifies initialization, but please verify that the updated processedFiles call fully covers all scenarios for lib resolution.

	p := &Program{opts: opts}