Move away from private serde APIs

[mischnic]

Part of #1056

Lifetime problem that I figured out already

error[E0309]: the parameter type `R` may not live long enough
   --> src/rules/mod.rs:436:56
    |
436 | #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize), serde(transparent))]
    |                                                        ^^^^^^^^^^^^^^^^^^ ...so that the type `R` will meet its required lifetime bounds
...
439 | pub struct CssRuleList<'i, R = DefaultAtRule>(
    |                        -- the parameter type `R` must be valid for the lifetime `'i` as defined here...
    |
    = note: this error originates in the derive macro `serde::Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider adding an explicit lifetime bound
    |
439 | pub struct CssRuleList<'i, R: 'i = DefaultAtRule>(
    |                             ++++

For more information about this error, try `rustc --explain E0309`.
error: could not compile `lightningcss` (lib) due to 1 previous error

[devongovett]

I think we may need to use a crate that provides similar functionality to the private types so that we can buffer correctly. e.g. serde_value

[devongovett]

Is it safe to assume that the type field always comes before the value field? I think that's why it was buffering before.

[devongovett]

See #1062