fix: Check for wasm-opt during configure & run on post_build

[phated]

Description

This PR makes the wasm-opt dependency more explicit. It is now referenced as a dependency since the wasm build fails without it. I've also added a find_program call to ensure it is installed during the configure phase for a wasm build; otherwise you'd get to the very end and it would crash.

I've changed the executable target to be barretenberg.wasm and then used the TARGET variant of add_custom_command so the wasm-opt command isn't run if barretenberg.wasm is already built. The previous way of doing this created a target that was always stale.

This should enable adding the wasm binary as an installation target, which is not possible with custom targets.

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.

[dbanks12]

Left a comment. Otherwise looks good to me. This is good cleanup.