Upgrade to syn v2

[SkymanOne]

• Bumps all syn dependencies to 2
• Attribute parsing has been reworked by @ascjones

[codecov-commenter]

Codecov Report

Merging #1731 (e042ade) into master (3f11e0e) will decrease coverage by 0.04%.
The diff coverage is 92.42%.

@@            Coverage Diff             @@
##           master    #1731      +/-   ##
==========================================
- Coverage   70.74%   70.71%   -0.04%     
==========================================
  Files         205      206       +1     
  Lines        6478     6454      -24     
==========================================
- Hits         4583     4564      -19     
+ Misses       1895     1890       -5     

Impacted Files	Coverage Δ
crates/ink/ir/src/ast/attr_args.rs	100.00% <ø> (+15.21%)	⬆️
crates/ink/ir/src/ir/item_impl/callable.rs	91.83% <ø> (-0.09%)	⬇️
crates/ink/ir/src/ir/item_impl/tests.rs	100.00% <ø> (ø)
crates/ink/ir/src/ir/storage_item/mod.rs	72.91% <ø> (ø)
crates/ink/ir/src/ir/trait_def/tests.rs	100.00% <ø> (ø)
crates/ink/ir/src/ir/trait_def/item/mod.rs	88.61% <83.33%> (ø)
crates/ink/ir/src/ast/meta.rs	85.48% <85.48%> (ø)
crates/ink/ir/src/ir/attrs.rs	80.76% <95.04%> (-1.26%)	⬇️
crates/ink/ir/src/ir/chain_extension.rs	93.33% <100.00%> (ø)
crates/ink/ir/src/ir/item/event.rs	87.83% <100.00%> (ø)
... and 7 more

... and 2 files with indirect coverage changes

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[ascjones]

LGTM :)