fix: lable + reference syntax (TOKEN_LABEL)

[Ziqi-Yang]

When writing my assignment today, I noticed that label and reference can starts with number, like <123>, @123. Typst Source Code

            '@' => self.ref_marker(),
            // ...
            _ => self.text(),

ref_marker appears before text and the definition of ref_marker uses only is_id_continue function.

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I'm busy with academic stuffs recent months. Sorry for lazy contribution.

[Ziqi-Yang]

Find error in this fix: it allows space inside label, <el psy> or @ el(reference only allows the space at the beginning), while official online application doesn't allow this rule.