Extract ManifestBuilder and EventListener

[agocke]

This reduces the size of the EventSource file, which I find helps reduce token usage

[Copilot]

Pull request overview

Refactors System.Diagnostics.Tracing implementation by extracting large helper types out of EventSource.cs into dedicated files, reducing the size of EventSource.cs while preserving behavior.

Changes:

• Moved ManifestBuilder implementation into ManifestBuilder.cs.
• Moved EventListener implementation into EventListener.cs.
• Updated System.Private.CoreLib.Shared.projitems to compile the new files.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/ManifestBuilder.cs	New file containing the extracted ManifestBuilder implementation.
src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventSource.cs	Removed in-file ManifestBuilder/EventListener definitions and kept remaining tracing types.
src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventListener.cs	New file containing the extracted EventListener implementation.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Adds the two new .cs files to the build.

[agocke]

/ba-g failure is #124417