[MOB-11503] Fix eslint errors on vanilla JS sample app

[pauljung14]

JIRA Ticket(s) if any

• MOB-11503

Description

• Fixes these linter errors

[eslint] 
/Users/paul.jung/dev/iterable-web-sdk/example/src/index.ts
   2:1   error  'axios' should be listed in the project's dependencies. Run 'npm i -S axios' to add it                 import/no-extraneous-dependencies
  10:8   error  Unable to resolve path to module '@iterable/web-sdk'                                                   import/no-unresolved
  16:46  error  Unexpected block statement surrounding arrow body; move the returned value immediately after the `=>`  arrow-body-style
  31:34  error  Unexpected block statement surrounding arrow body; move the returned value immediately after the `=>`  arrow-body-style
  53:32  error  ["ExternalNewTab"] is better written in dot notation                                                   dot-notation
  63:31  error  ["Deferred"] is better written in dot notation                                                         dot-notation

• Also adds support for using env variables for login email and jwt generator url.

Test Steps

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.85%. Comparing base (8a4c1c0) to head (87455ef).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #501   +/-   ##
=======================================
  Coverage   64.85%   64.85%           
=======================================
  Files          47       47           
  Lines        1787     1787           
  Branches      567      567           
=======================================
  Hits         1159     1159           
+ Misses        627      588   -39     
- Partials        1       40   +39     

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