Get the current source cursor, to be restored later with restoreSourceCursor.
This is useful when analyzing source code out-of-order.
fn saveSourceCursor(astgen: *const AstGen) SourceCursor
fn saveSourceCursor(astgen: *const AstGen) SourceCursor {
return .{
.offset = astgen.source_offset,
.line = astgen.source_line,
.column = astgen.source_column,
};
}