Dotnetup: add env command to update PathPreference mode

[dsplaisted]

Currently, this PR includes the changes from #54520 but adds a design doc for how to address #53742.

Implementation will be added later, probably to this PR.

Basically, this replaces the defaultinstall command with an env command.

Proposal

Replace dotnetup defaultinstall <user|system> with a noun-verb env command family
backed by a renamed three-value PathPreference enum:

dotnetup env set <none|shell|all>   # persist mode in config and apply
dotnetup env show                   # display current mode; if applied state has
                                    # drifted from the configured mode, report that
                                    # too
dotnetup env script                 # print a shell-specific script that exports
                                    # PATH/DOTNET_ROOT for the current dotnet
                                    # install (formerly `dotnetup print-env-script`)

env set is idempotent — running it again with the same mode is a no-op (and is also
how you re-sync after something else has clobbered your PATH; see the key scenario
below).

Modes:

• none — Don't modify your environment; use dotnetup dotnet to invoke.
• shell — Wire dotnetup into your shell's profile file (only shells that load
  profiles see the user dotnet).
• all — Wire dotnetup into your shell profile and your user-level
  PATH/DOTNET_ROOT so cmd.exe, IDEs, and shortcuts also see it.

See the design doc for more.