feat(update): add mechanism to allow updating/writing back to `package.json`

[darcyclarke]

Problem Statement

We've experience significant feedback from users that they were using the npm update command to reflect changes in package.json; Rightly or wrongly, this seems to be a pattern which is useful for some & we should facilitate a mechanism for them to achieve this in v7

[darcyclarke]

References:

• [BUG] npm update not updating package.json cli#708
• Search: https://github.com/npm/cli/issues?q=is%3Aissue+is%3Aopen+%22npm+update%22+%2B+%22save%22+sort%3Acomments-desc+

Example of what we could do... as a feature

• Have npm update support --save which indicates that you want to save the latest spec back to package.json

npm update foo --save

Current Status/Expectations/Potential

• package-lock=true & save=false are the defaults
• package-lock=true + save=true = package.json is updated / package-lock.json is updated
• package-lock=false + save=true = package.json is updated / package-lock.json isn't updated
• package-lock=false + save=false = package.json isn't updated / package-lock.json isn't updated

[ruyadorno]

Landed in npm/cli#4223