Add JS functions, parameters, arrow functions and IIFEs (#1661)

* Add javascript functions

* Add javascript function parameters

* Add javascript arrow functions

* Add javascript IIFE

* Update content/roadmaps/106-javascript/content/109-javascript-functions/101-function-parameters/readme.md

* Update content/roadmaps/106-javascript/content/109-javascript-functions/102-arrow-functions.md

* Update content/roadmaps/106-javascript/content/109-javascript-functions/103-iifes.md

* Update content/roadmaps/106-javascript/content/109-javascript-functions/readme.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/106-javascript/content/109-javascript-functions/101-function-parameters/readme.md

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-# Function parameters
+# Function Parameters
+
+The parameter is the name given to the variable declared inside the definition of a function. There are two special kinds of syntax: default and rest parameters.
+
+<BadgeLink colorScheme="yellow" badgeText="Read" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#function_parameters">Function Parameters</BadgeLink>
+

content/roadmaps/106-javascript/content/109-javascript-functions/102-arrow-functions.md

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-# Arrow functions
+# Arrow Functions
+
+Arrow Function is a new way of creating functions with the '=>' operator with a shorter syntax.
+
+<BadgeLink colorScheme="yellow" badgeText="Read" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions">Arrow function expressions</BadgeLink>

content/roadmaps/106-javascript/content/109-javascript-functions/103-iifes.md

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-# Iifes
+# IIFE
+
+Immediately-Invoked Function Expression is a function that is executed immediately after it is created.
+
+<BadgeLink colorScheme="yellow" badgeText="Read" href="https://developer.mozilla.org/en-US/docs/Glossary/IIFE">IIFE — MDN Docs</BadgeLink>

content/roadmaps/106-javascript/content/109-javascript-functions/readme.md

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-# Javascript functions
+# Functions
+
+Functions are blocks of code that execute whenever they are invoked. They are useful for placing code snippets executed in different places in the code.
+
+<BadgeLink colorScheme="yellow" badgeText="Read" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions">Functions - MDN Docs</BadgeLink>