build(deps): bump syn from 2.0.66 to 2.0.68

[dependabot[bot]]

Bumps syn from 2.0.66 to 2.0.68.

Release notes

Sourced from syn's releases.

2.0.68

• Improve panic location when parse_quote! parses invalid syntax (#1690, thanks @​stepancheg)
• More efficient peek implementation for Group and Lifetime (#1687)

2.0.67

• Produce more accurate error message locations for errors located at the end of a nested group (#1679, #1680)
• Support peeking LitCStr in ParseStream::peek (#1682)

Commits

• ecb0429 Release 2.0.68
• 37edbd2 Merge pull request #1690 from stepancheg/track-called
• 4338772 track-caller in parse_quote_spanned
• 537b0c5 Update test suite to nightly-2024-06-22
• b088d5c Merge pull request #1688 from dtolnay/peeklit
• 9b4e478 Relocate Lit peek impls into lit module
• 38f2ddb Merge pull request #1687 from dtolnay/fastpeek
• ef20bfd Optimize the peek impl of Group and Lifetime
• 4b6c96a Merge pull request #1686 from dtolnay/isempty
• 87d5792 Reword explanation of ParseBuffer::is_empty
• Additional commits viewable in compare view

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