feat: support for elan 4.0.0

[mhuisi]

This PR implements support for Elan 4.0.0. Specifically:

• A new setting that can be enabled to issue confirmation prompts before installing new toolchains
• Confirmation prompts before eagerly updating release channels when opening Lean files in VS Code with Elan 4.0.0's eager toolchain resolution
• New VS Code command integrations for Elan:
  • Project and default toolchain selection that displays all available Lean toolchains by using https://release.lean-lang.org/
  • Toolchain deinstallation, including integration for elan toolchain gc in Elan 4.0.0
  • Manual updates of release channel toolchains

It also fixes a couple of bugs:

• When the output of elan show got too large, the 'Troubleshooting: Show Setup Information' command would display a dialog that could exceed the height of the screen, hiding all buttons. With Elan 4.0.0, the content of this dialog is now kept much smaller so that this cannot happen on most screens anymore.
• The InfoView would sometimes report internal errors (including stacktraces) when something went wrong on the VS Code side of the InfoView. Now it correctly displays the error.
• The order of the buttons in lots of modal dialogs was a bit unorganized, and it should now be more coherent.
• If the default toolchain was changed while VS Code was running, the VS Code extension would hold on to the old toolchain for the language client of untitled files indefinitely.
• VS Code extension commands that use the active client of the client provider would behave incorrectly or use an incorrect client if a client failed or was stopped manually using a command.
• The 'Restart Server' command didn't always restart the setup check for the selected language client if that language client had failed in the past.

Update dialog

Installation dialog

New version management commands

Default version dialog

Release channel update dialog

Version uninstall dialog

This PR will be merged at the start of 2025.