feat(css_formatter): Always quote attribute matcher values

[faultyserver]

Summary

#1285. This PR ensures that all values for an attribute selector are quoted. For example:

[type=text][type="checkbox"] {}

/* becomes */
[type="text"][type="checkbox"] {
}

The quotes are not necessary syntactically, but ensure consistency across the board (and this matches Prettier's output).

Once quote styles are supported, this will also select the appropriate quote character to use, but that is a future situation.

Test Plan

The attribute matching spec test is updated with quoted values.

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