Function format_source 

pub fn format_source(source: &str) -> FmtResult

Formats a single Rust source file by reformatting all euv macro invocations.

Scans the source code for html!, class!, vars!, and watch! macro calls, then applies formatting rules to their token bodies:

• Tag name and { separated by exactly one space
• Attribute name immediately followed by :, then one space before the value
• if { expr } with single spaces around each token
• match { expr } with proper alignment
• for pattern in { expr } with proper alignment
• => in watch! macros with proper spacing

Arguments

• &str - The Rust source code content.

Returns

• FmtResult - The formatting result indicating whether changes were made.