Auto merge of #12212 - emilio:stylo-regen-script, r=bholley

stylo: Move all binding-generator logic code to a python script.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

<!-- Either: -->
- [x] These changes do not require tests because tooling

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

---

This not only makes us more consistent with the rest of the codebase but also:

 * Makes us repeat less code like common flags and all that stuff.
 * Reduces the noise of the build: You only get the output of the commands on
   failure or when you pass the -v flag.
 * Makes you able to select a single kind of build or multiple in the same
   place.

I've basically kept the regen.sh script because of the LIBCLANG_PATH checks, but
at least from Linux I don't need them anymore. Also, that logic could be moved
to the new script.

The whole point of this isn't only making it prettier and easier to use, but
also allowing me to write more complex logic in the binding generator scripts,
that I will probably need to integrate the DOM enum types we need for animations
and such easily (can't be just an include, because that pulls in another header
with the same name bringing a lot of DOM and IDL churn).

For reference, here's a successful regen round with the script:

```
./regen.py --target all /home/emilio/projects/moz/stylo/gecko/obj-x86_64-pc-linux-gnu/
[BINDGEN] structs::release in "/home/emilio/projects/moz/stylo/gecko/obj-x86_64-pc-linux-gnu/"... OK
[RUSTC]... OK
[RUSTC_TEST]... OK
test result: ok. 168 passed; 0 failed; 0 ignored; 0 measured
[BINDGEN] structs::debug in "/home/emilio/projects/moz/stylo/gecko/obj-x86_64-pc-linux-gnu/"... OK
[RUSTC]... OK
[RUSTC_TEST]... OK
test result: ok. 169 passed; 0 failed; 0 ignored; 0 measured
[BINDGEN] bindings::None in "/home/emilio/projects/moz/stylo/gecko/obj-x86_64-pc-linux-gnu/"... OK
```

---

r? @bholley

Maybe @jgraham and/or @Wafflespeanut want to take a look to criticize my (lack of) pythonism ;-)

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12212)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions