Add tool_version_manager.enabled config setting

Doug Barrettrequested to merge
tool-version-manager-opt-out-config into main

What does this merge request do and why?

To aide in a smooth transition away from asdf,

This MR introduces a new config setting to eventually replace current usages of mise.enabled and asdf.opt_out scattered throughout the codebase.

The intent of this boolean config variable tool_version_manager.enabled? is to allow users to self-manage their tool versions.

See relevant discussion: &19027 (comment 2697612623) !5138 (comment 2707221148) !5138 (comment 2706105202)

Closes #2916 (closed)

Edited by Doug Barrett

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