Fixed case-insensitive name check in checkSketchFile

[magedrifaat]

This commit fixes #8030

It turns out that on case-insensitive file systems, the "file.exists" check will return true even if the file that actually exists differs from the file being checked in the case of the letters. (i.e. "Test.ino" will be considered to exist even if the actual file name is "test.ino").
The proposed solution is to make use of the function getCanonicalFile as suggested in this StackOverflow answer to get the actual file name and compare it to the expected name.

I decided to implement a function in FileUtils for better code reuse but it could have been inlined as well since this is its only use-case currently.

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