Add glossary/Selector

Also capitalized all the glossary entries.

glossary/Selector.md

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+### Selector
+
+A CSS selector is a pattern that is used to select and/or style one or more elements in a document, based on certain rules.
+The order in which CSS selectors apply styles to elements is based on the rules of CSS specificity.

glossary/array.md

@@ -1,4 +1,4 @@
-### array
+### Array
 
 Arrays are used to store multiple values in a single variable.
 Arrays are ordered and each item in an array has a numeric index associated with it.

glossary/callback.md

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-### callback
+### Callback
 
 A callback function, also known as a high-order function, is a function that is passed into another function as an argument, which is then executed inside the outer function.
 Callbacks can be synchronous or asynchronous.