[WIP] WASM build for qiskit-cext

[s5bug]

Summary

See also Qiskit/qiskit-rs#16

This adds a wasm_js feature to qiskit-cext that enables the correct features in the different versions of getrandom pulled in transitively.

Details and comments

I'm not a Cargo expert: this could be a completely terrible way to do this. I can also tell that my changes are missing proper "why is this here" comments. I also also know that I should probably add documentation to this somewhere, but I don't know where or in what manner.

I have no idea how to test WASM at all (and it doesn't yet build on my machine because I lack a recent Python).

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 20569468629

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 25 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 88.3%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	91.26%
crates/circuit/src/parameter/symbol_expr.rs	8	72.98%
crates/qasm2/src/parse.rs	12	96.62%

Totals
Change from base Build 20366546417:	-0.01%
Covered Lines:	96704
Relevant Lines:	109517

---

💛 - Coveralls