WorkspaceBuilder: make the classic builder selectable by dotted path

[tony]

Summary

Add the first implementation step from #1058: make the current builder the explicit classic/default builder and let users select a workspace builder by Python dotted path.

Related: #1053, #1056, #1058.

Proposed behavior

• Preserve current load behavior unless a workspace explicitly opts in.
• Treat the current builder as the classic builder and keep it as the default.
• Add a workspace_builder config key with default tmuxp.workspace.builder.WorkspaceBuilder.
• Add an optional workspace_builder_paths list for trusted user-controlled import roots when tmuxp's runtime environment does not already see the builder package.
• Resolve workspace_builder_paths by expanding ~ and environment variables, resolving relative paths relative to the workspace file, requiring entries to be directories, and temporarily prepending them to sys.path for the builder import/build duration.
• Do not use site.addsitedir() for workspace-config paths; site-directory processing can execute startup files and is broader than making a module importable.
• Validate the imported object is a builder class or compatible builder before use.
• Keep pane-readiness policy and parallel behavior out of this PR.

Example shape:

workspace_builder: my_tmuxp_builders.CustomBuilder
workspace_builder_paths:
  - ~/.config/tmuxp/builders

Acceptance criteria

• Existing workspace files load through the classic default unchanged.
• A workspace can select a builder by dotted path.
• A workspace can add explicit trusted import roots without .pth or site startup processing.
• Import errors are actionable and do not leak machine-specific details.
• Tests cover default selection, dotted path selection, path-assisted selection, missing path failure, and invalid target failure.

[tony]

Shipped in v1.73.0 (#1066). A workspace can now select a builder via the workspace_builder key — a Python import path (package.module:Builder or module.attr) or a registered entry-point name. The built-in classic builder (ClassicWorkspaceBuilder, registered as classic) stays the default, so existing workspaces are unaffected.