Fix Greaselion's precondition detection

[szilardszaloki]

Description

1. Greaselion.json should get reparsed on the fly as preconditions defined in it get fulfilled. Currently, it's parsed once, at browser startup. This results in the issue mentioned here.
2. The precondition of youtubeBase.ts is "rewards-enabled": true, but it even loads with Rewards being turned off:
   2.1. new profile
   2.2. visit youtube.com
   2.3. Greaselion script loaded: youtubeBase.ts appears in developer console
   This results in quite a couple of instances of this:

[3700:9420:0204/140236.445:ERROR:CONSOLE(21770)] "Failed to handle media duration metadata: missing publisher key for media key youtube_3jS_yEK8qVI", source: chrome-extension://mnojpmjdmbbfmejpflffifhffcmidifd/out/brave_extension_background.bundle.js (21770)

Miscellaneous Information:

cc: @Miyayes @brave/rewards-client