Vitest issue fix poc

[mikavilpas]

This PR demonstrates that #127 can indeed be solved by changing the name of the then combinator to something else. Here I purposefully chose the horrible name thenceforth (wiktionary 🙂) to indicate that a better name should be come up with.

It's a bit difficult to demonstrate the fix because of the setup. Let me describe how I verified that it works:

1. clone the issue reproduction repo https://github.com/katangafor/parjs-vitest-error
2. I used use nodejs 20.9.0 (others might work too, haven't tried)
3. setup a local link to the parjs monorepo using this diff (might need to vary the path):

Details

diff --git a/package.json b/package.json
index 4686781..4a9ef19 100644
--- a/package.json
+++ b/package.json
@@ -15,6 +15,6 @@
     "vitest": "^0.34.6"
   },
   "dependencies": {
-    "parjs": "^0.16.1"
+    "parjs": "file:../parjs/packages/parjs/"
   }
 }

4. Check out this PR's branch in the parjs monorepo and issue yarn && yarn build (might need to do it twice, but it will work)
5. in the issue reproduction repo, issue yarn && yarn vitest to see that the test passes and does not hang vitest indefinitely

To reproduce the issue and see that the test hangs without the fix, do these steps:

1. in the parjs monorepo
   • revert the fix using git revert
   • run yarn build (maybe twice)
2. in the issue reproduction repo
   • run yarn for good luck I guess (might not be needed, haven't tried)
   • run the test again with the command above. It should now hang forever because the fix in parjs has been reverted.