feat: append '*.parser' to file extensions

[zhylmzr]

Previously, if I wanted to format new extension files like ".wxml" using Prettier, I had to use the following configuration

"prettier": {
    "associations": [
        "**/*.{js,ts,less,json}",
        /** and any file extension that you want to format and is supported by Prettier.  */
        "**/*.wxml"
    ],
    "wxml.parser": "html"
}

After this feature, there is no need to manually configure the 'associations' anymore.

"prettier": {
    "wxml.parser": "html"
}

fixed: dprint/dprint#841

[CLAassistant]

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