`rtx doctor` improvements

[jdx]

if users report a problem, we should have them run rtx doctor first and display the output.

Some things this should probably display:

• [ ] rtx version
• [ ] rtx os/arch
• [ ] rust version used to compile the build
• [ ] operating system version? not sure how easy this is to get but might be good to have
• [ ] current shell and version
• [ ] RTX environment variables
• [ ] RTX settings
• [x] active tool-version and legacy version files
• [ ] active plugins with repo url and git sha (maybe denote if these are old/out of date somehow)
• [ ] active runtimes
• [ ] add rtx doctor instructions to GH issue template