fix(codegen): inline function expression can be named inside a v-on…

[Justineo]

… expression (fix #9707)

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
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Does this PR introduce a breaking change? (check one)

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• No

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Other information:

[posva]

Looks good. We could also forbid \d as the first character of the identifier if we want to be strict

[Justineo]

I wanted to use [A-Za-z_$][\w$] at first but switched to [\w$_] to be consistent with the earlier part of the same regexp. Not sure if we should make them both stricter.