Fix: eslint.verify should not mutate config argument (fixes #8329)

[alberto]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[X] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What parser (default, Babel-ESLint, etc.) are you using?
default

Please show your full configuration:
N/A

What did you do? Please include the actual source code causing the issue.

const eslint = require("eslint");

const config = {};

eslint.linter.verify("foo", config);

console.log(config);

What did you expect to happen?
I expected config to still be an empty object.

What actually happened? Please include the actual, raw output from ESLint.
A globals property was added to config, so the output was:

{ globals: {} }

What changes did you make? (Give an overview)
Avoid modifying the argument object. Fixes #8329

Is there anything you'd like reviewers to focus on?
No.

[eslintbot]

LGTM

[mention-bot]

@alberto, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mysticatea, @not-an-aardvark and @nzakas to be potential reviewers.

[ilyavolodin]

@alberto Could you do perf run before and after this change?

[alberto]

@ilyavolodin no noticeable difference:

Before:

Loading:
  Load performance Run #1:  174.65021ms
  Load performance Run #2:  168.799739ms
  Load performance Run #3:  178.816711ms
  Load performance Run #4:  177.747853ms
  Load performance Run #5:  182.442309ms

  Load Performance median:  177.747853ms


Single File:
  CPU Speed is 2600 with multiplier 13000000
  Performance Run #1:  4207.118519ms
  Performance Run #2:  4098.461712ms
  Performance Run #3:  3699.8055409999997ms
  Performance Run #4:  3638.281188ms
  Performance Run #5:  3681.332236ms

  Performance budget ok:  3699.8055409999997ms (limit: 5000ms)


Multi Files (0 files):
  CPU Speed is 2600 with multiplier 39000000
  Performance Run #1:  12933.732048ms
  Performance Run #2:  12714.120701ms
  Performance Run #3:  12723.325213ms
  Performance Run #4:  12861.724682ms
  Performance Run #5:  12863.99631ms

  Performance budget ok:  12861.724682ms (limit: 15000ms)

After:

Loading:
  Load performance Run #1:  174.81977ms
  Load performance Run #2:  179.599493ms
  Load performance Run #3:  183.244189ms
  Load performance Run #4:  181.803644ms
  Load performance Run #5:  165.226383ms

  Load Performance median:  179.599493ms


Single File:
  CPU Speed is 2600 with multiplier 13000000
  Performance Run #1:  3713.787855ms
  Performance Run #2:  3659.128527ms
  Performance Run #3:  3659.855329ms
  Performance Run #4:  3786.704485ms
  Performance Run #5:  4434.773668ms

  Performance budget ok:  3713.787855ms (limit: 5000ms)


Multi Files (0 files):
  CPU Speed is 2600 with multiplier 39000000
  Performance Run #1:  12947.623801ms
  Performance Run #2:  12859.656538ms
  Performance Run #3:  12722.837927ms
  Performance Run #4:  12703.583254ms
  Performance Run #5:  13003.146405ms

  Performance budget ok:  12859.656538ms (limit: 15000ms)