Sync format of custom messages with that in the Stylelint configurations

[jhae-de]

Using the stylelint-config-standard configuration as an example, the messages of the *-pattern rules have the following format:

Expected container name "${containerName}" to be kebab-case

I suggest adjusting the messages in this configuration accordingly.

Rule	Message
scss/at-function-pattern	Expected function name "${functionName}" to be kebab-case
scss/at-mixin-pattern	Expected mixin name "${mixinName}" to be kebab-case
scss/dollar-variable-pattern	Expected variable name "${variableName}" to be kebab-case
scss/percent-placeholder-pattern	Expected placeholder name "${placeholderName}" to be kebab-case

Currently, however, these rules do not support passing arguments to custom messages, so the changes could therefore only be made once #1186 has been resolved.