feat(pkg-config): Add a bindir variable

[phated]

Description

This adds a bindir variable to the pkg-config file, so we can use the tool to find the location where the wasm file will be installed—such that you can do ls $(pkg-config --variable bindir barretenberg)/barretenberg.wasm to locate the wasm file.

This is useful for rust build scripts to locate the wasm file.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR specifications in /markdown/specs have been updated.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• I've updated any terraform that needs updating (e.g. environment variables) for deployment.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.

[kobyhallx]

👍 LGTM