npm-whoami: add page

[canonnizq]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known): 10.9.0

[CLAassistant]

All committers have signed the CLA.

[tldr-bot]

The build for this PR failed with the following error(s):

pages/common/npm-whoami.md:
Error: Parse error on line 4:
...pmjs.com/cli/whoami>- Display npm user
-----------------------^
Expecting 'END_INFORMATION_LINK_URL', got 'NEWLINE'
pages/common/npm-whoami.md:4: TLDR011 Page never contains more than a single empty line

Please fix the error(s) and push again.

[spageektti]

Hi, thanks for your contribution.

[spageektti]

lgtm