Add a "this" method to ContractId

[SwayStar123]

Description

Adds a ContractId::this() method, equivalent to std::call_frames::contract_id(). For ease of use.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
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[K1-R1]

Late to the discussion, but imo the best syntax is:
let this_contract = contract_id();
I would instead advocate for the removal of the function contract_id() from call_frames and for this new function in the contract_id lib to be renamed contract_id()