Add intercept service to wdio

[chmanie]

Proposed changes

Add the wdio-intercept-service to the cli and docs. It's already linked on the website (https://webdriver.io/docs/wdio-intercept-service.html)

Types of changes

• Documentation update
• CLI plugin addition

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (not applicable)
• I have added necessary documentation (if appropriate)
• I have added proper type definitions for new commands (if appropriate)

Reviewers: @webdriverio/technical-committee

[jsf-clabot]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #4976 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4976   +/-   ##
=======================================
  Coverage   99.41%   99.41%           
=======================================
  Files         210      210           
  Lines        5504     5504           
  Branches     1178     1178           
=======================================
  Hits         5472     5472           
  Misses         29       29           
  Partials        3        3

Impacted Files	Coverage Δ
packages/wdio-cli/src/constants.js	100% <ø> (ø)	⬆️

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[christian-bromann]

some minor things

[christian-bromann]

👍