Unquoted text on stable

[vldm]

Currently RawText uses unstable Span::join https://github.com/rs-tml/rstml/blob/v0.10.1/src/node/raw_text.rs#L52
to provide source text with spaces. Because of its unstability it is not availabe on stable rust.
I found that in test https://github.com/rs-tml/rstml/blob/v0.10.1/tests/test.rs#L110 unquoted text is working even on stable, because fallback implementation of proc_macro2::TokenStream can handle Span::join.

Because Unquoted text is important feature in HTML/XML context, we can try to reuse proc_macro2::Span::join.

Proposal:
Implement temporary workaround that will use proc_macro2 fallback implementation directly.

Parse two times:
One with regular TokenStream, and with correct unjoined Span.
And second time with fallback::TokenStream created from Span::call_site().source_text().

Both parse phases should return same Node tree, but with different Spans.
Use second Node tree to recover source_text of RawText.

Cons:

• Parse two time

[vidhanio]

for the cons, maybe there could be a feature flag you could add to enable re-parsing? this would be useful for people who don't care about having <style>/<script> support.