Changed the parsing of custom block to parse like Vue3 parser.

[ota-meshi]

This PR changes the custom blocks to parse as RAWTEXT.

It parses like the Vue 3 SFC parser.
However, if the file name is other than .vue such as .html, the custom block will not be parsed as RAWTEXT.

In the following edge case, there is a difference between the parsed result and the one before the change.

<custom-block>
  End Tag in the Attr. e.g. <foo attr="</custom-block>" ></foo>
</custom-block>

The content of custom block is
End Tag in the Attr. e.g. <foo attr="</custom-block>" ></foo>
in Vue2, but
End Tag in the Attr. e.g. <foo attr="
in Vue3.
With this PR change, the parser will get the same parsing results as Vue3.

close #79