Upgrade to swc_core 0.91.0

[JonathanUsername]

As requested in #27 I try to address the required upgrade.

• Upgrade to 0.91.0
• Need to upgrade the rust-toolchain version to latest stable to handle proc_macro_span
• Convert now missing PatOrExpr to AssignTarget (this is NAIVE, I am trusting tests here)
• convert tests to test_inline as per other upgrades in swc

[AndrewIngram]

0.91 might be a hop too far in one go, as the broader ecosystem is currently on 0.90

I've tried this out but with 0.90 instead, and it seems to work with Next 14.2

[JonathanUsername]

You can probably leave the serde changes too, I don't think they're necessary