Add a __post_init hook, general updating and cleanup

[jessebraham]

• Adds a __post_init hook, like we have in esp-riscv-rt
• Updates dependencies to their latest versions
  • Updated syn from 1.x to 2.x in the proc-macro crate
• Bumped version numbers of xtensa-lx-rt-proc-macros and xtensa-lx-rt packages in preparation for new release
• Added a rust-toolchain.toml file

Sorry for the other noise, just trying to get the Cargo manifests consistent with the others. Additionally I bumped the MSRV to 1.65 since that's what we're using for the PACs/HALs anyway.

[bjoernQ]

I think we don't use it anyway but in xtensa.in.x we probably want to add

PROVIDE(__post_init = default_post_init);

or maybe we want to get rid of xtensa.in.x completely ... not sure

[bjoernQ]

besides the thing about xtensa.in.x this looks good to me

[jessebraham]

Okay I've updated the linker script for now, guess we can discuss whether or not to remove the linker script at another time.

Thanks for the review!