refactor: merge IexecERC20Base into IexecEscrowTokenFacet

[zguesmi]

Merges the IexecERC20Base abstract contract into IexecEscrowTokenFacet, its only implementer.
The facet ABI is identical before and after.

Next step

Rename:

• IexecEscrowTokenFacet -> IexecEscrowFacet
• IexecEscrowToken (interface) -> IexecEscrow
• IexecInterfaceToken -> IexecInterface

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (26772eb) to head (8e3fa58).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #345   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          32       31    -1     
  Lines        1095     1095           
  Branches      223      212   -11     
=======================================
  Hits         1090     1090           
  Misses          5        5           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.