repl: warn on assignment to _

[cjihrig]

Pull Request check-list

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with
  this change (including linting)?
• Is the commit message formatted according to [CONTRIBUTING.md][0]?
• If this change fixes a bug (or a performance problem), is a regression
  test (or a benchmark) included?
• Is a documentation update included (if this change modifies
  existing APIs, or introduces new ones)?

Affected core subsystem(s)

repl

Description of change

The REPL uses _ to store the result of the previous expression. This can cause confusion for REPL users who assign libraries like Underscore and Lodash to _. The documentation already mentions this behavior. This commit adds a printed warning if the user assigns to _.

Refs: #5431

[rvagg]

How does this interact with #3737, is it one or the other?

[cjihrig]

@rvagg it looks like this subsumes #3737. I'm also seeing a few different behaviors, depending on the type of variable:

• const - In master, const _ = 3; throws a TypeError trying to assign a read only property. In repl: assign underscore fix #3737 and this PR, a different TypeError is thrown that _ is already declared.
• let - In master, repl: assign underscore fix #3737, and this PR, a let variable takes over _ completely.
• var or global - This PR displays a warning on assignment. repl: assign underscore fix #3737 and master silently assign.

I think the varying behavior is related to #983.

[silverwind]

Can't say I like this approach. I'd rather see something like #5431 (comment)