Add way to lock a Manifest.toml

[IanButterworth]

I think it would be useful/reassuring to add support for an optional locked=true field to a Manifest.toml which Pkg would respect.
Specifically including the sandbox resolve that Pkg.test does.

When locked=true :

• No packages can change
• No packages can be added
  • Special allowance for Pkg.test to add packages given it's creating a new environment, but main manifest must be maintained (same as setting allow_reresolve=false)

[DilumAluthge]

I think this would be very useful.