Create devcontainer.json

[Loony3]

Description

Please provide a paragraph or two giving a summary of the change, including relevant motivation and context.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• There are no unexpected formatting changes, or superfluous debug logs.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewers next convenience.
• NatSpec documentation of all the non-test functions is present and is complete.
• Continuous integration (CI) passes.
• Command forge coverage --match-contract MyContract returns 100% line coverage.
• All the possible reverts are tested.

[Loony3]

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