Fix settings not available bug and adding test suite

[barrymun]

Description

• Sometimes (this is a rare but definitely problematic bug) the settings may not be available in the content and popup scripts, as the background script did not set the values before they were read
• the goal of this pr is to ensure that the settings are only set in one place (for both local and sync storage) to avoid confusion and to guarantee one version of settings is set in the background script, and that retry logic exists in the popup and content scripts so that the settings are always retrieved (unless there is a problem fetching the brands list (this should be addressed in another pr maybe))
• adding test suite (using jest lib)

Changes

• refactor the background so that the storage values are only set in one place (for both local and sync)
• setStorageValue function now uses function overloading to prevent certain values being set in sync storage
• rework the types slightly, update IIFEs in content and popup scripts
• add lodash lib and corresponding types
• setTimeout changed to setInterval so that the brand list update check function is run every day, not just once a day later
• fix a bug in the mutation observer whereby new items added to the page (Amazon may not add all items to the page immediately) were not picked up and were still shown even though they should be hidden
• "brandsMap", "brandsCount", "brandsVersion" and "maxWordCount" will no longer be set in sync settings, only local settings. it is impossible to set "brandsMap" in sync settings in chrome anyway because of sync storage size restrictions
• separate browser-side helper functions from general helper functions in the utils directory (this is helpful to set up jest quickly)
• adding a new test suite (jest). the command yarn test will run the tests
• tests are run automatically on commit as it has been added to the pre-commit hooks
• implement the getSettings function which will first check for sync settings but fall back to local if sync settings are not available for whatever reason
• better type support with function overloading for getStorageValue and setStorageValue functions
• add a mode to setStorageValue function, where if override is set it will clear all values in storage before setting the new value. normal is the default setting
• rename prop param to storageArea for improved readability
• rename Storage<> types for improved readability

[barrymun]

@chris-mosley there are some nice changes in this pr; i've tried to document all of the alterations but please have a look through the description.

the background script has been given a big rework so that we only set the storage values once to avoid race conditions where some (or all) of the values may not actually exist in storage. i've also improved the typing around the getStorageValue and setStorageValue functions so it should tell us if we're trying to set values in storage that should not be set (for example, we should not set brandsMap in sync storage as this will result in errors for chrome extensions).

i've also included the basic work for a test suite which can be expanded on to make the code more resilient in the future too.

i would appreciate it if you could test this one before merge when you get a chance, as there are a lot of changes. if there are any issues, or you spot any problems when using the extension/addon, please let me know and i can try to fix before we merge. if you have any other questions please don't hesitate to ask. i've tested as best i can and everything seems to work correctly on my end, but it's always good to get another pair of eyes on larger prs. thanks

[chris-mosley]

Did a fair amount of testing on this and it seems to be working. I did uninstall and reload it a few times but i believe when i do that I get a new guid for the addon, so I'm not certain that it really tested overwriting of old settings or not.