Extract custom watch paths to specifications

[isaacroldan]

WHY are these changes introduced?

The current file watching system for dev sessions uses a hardcoded approach where function extensions have custom watch paths while other extensions watch everything. This creates inconsistent behavior and makes it difficult for different extension types to define their own watching requirements.

WHAT is this pull request doing?

Refactors the dev session file watching system to use a specification-based approach:

• Replaces devSessionCustomWatchPaths with a new devSessionWatchConfig method on extension specifications
• Introduces a DevSessionWatchConfig interface that allows extensions to define both paths to watch and patterns to ignore
• Moves function-specific watch logic from the base ExtensionInstance class to the function specification
• Adds watch configuration for admin extensions that only watches the static_root directory when configured
• Sets default behavior where 'configuration' experience extensions watch nothing and 'extension' experience extensions watch everything
• Moves default ignore patterns from the file watcher service to the extension instance level

How to test your changes?

1. Create a function extension with custom build.watch configuration and verify it only watches the specified paths plus default files (locales, GraphQL, TOML)
2. Create an admin extension with static_root configured and verify it only watches files in that directory
3. Create other extension types and verify they follow the default watching behavior based on their experience type
4. Run dev sessions and confirm file changes trigger rebuilds appropriately for each extension type

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing, so I've added a changelog entry with pnpm changeset add

[isaacroldan]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Extract custom watch paths to specifications #7220 : 2 dependent PRs (#7226 , #7227 ) 👈 (View in Graphite)
• Use extension handle to identify extensions in file-watcher #7224
• main

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