Semgrep linters

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Linting rules for use with `semgrep`_ during CI checks on PRs.

Status

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This is an experimental approach to developing new linting rules. Semgrep provides by-example structural matching that can be easier to write and maintain than procedural code inspecting ASTs. If the approach works out, we can expand our use of Semgrep; if it becomes a problem for some reason, we can switch to adding pylint rules in edx-lint.

Ignoring failures

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If you need to tell semgrep to ignore a block of code, put a ``# nosemgrep`` comment on or before the first matched line.

Documentation for writing new rules:

- https://semgrep.dev/docs/writing-rules/rule-syntax/

- https://semgrep.dev/docs/writing-rules/pattern-syntax/

.. _semgrep: https://github.com/returntocorp/semgrep