Update syn requirement from 0.15 to 2.0

[dependabot[bot]]

Updates the requirements on syn to permit the latest version.

Release notes

Sourced from syn's releases.

2.0.4

• Improve error reporting when parsing identifiers and paths (#1415, #1416)

Commits

• 91ca7c0 Release 2.0.4
• e7bb238 Merge pull request #1416 from dtolnay/patherrors
• 7bb31b1 Improve error reporting in path parser
• eaf6215 Merge pull request #1415 from dtolnay/identkeyword
• 18cac49 Improve error message when parsing Ident encounters keyword
• 25def51 Release 2.0.3
• 4f3803d Merge pull request #1412 from dtolnay/exprgroup
• 516a5b5 Provide Expr::Group even with features="full" off
• cac5cc6 Release 2.0.2
• 8f826ef Touch up spacing in example for parse_multi_with_leading_vert
• Additional commits viewable in compare view

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