default modifier: support function-call syntax and quoted string literals

[Copilot]

SQL function calls like NOW() should not require quoting in default(...). String typed values should carry their quotes in the AST. Two related fixes:

Parser

• Added TOKEN_STRING case — stores value with surrounding double quotes (e.g. "active")
• Added function-call detection for IDENT — when followed by (), stores as NOW() instead of requiring "NOW()"

Usage

created_at timestamp not-null default(NOW())   # function call — no quotes needed
status     varchar   not-null default("draft") # string literal — quotes preserved
sort_order int       not-null default(0)       # number

Updated

• All example .erdn files: default("NOW()") → default(NOW())
• website/syntax.md, website/guide.md, README.md: docs and examples updated to reflect both forms; syntax description now mentions function calls as a supported value kind
• Parser tests: added TestParseDefaultFunctionCall; updated TestParseDefaultModifier to use quoted string form

[github-actions]

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