Refactor extend statement: remove unnecessary "as" keyword and push "this" variable context to the extend-statement scope

modules/collection/src/main/resources/list.mus

@@ -14,7 +14,7 @@ function _flatten(list: list, output: list) {
     return output;
 }
 
-extend list as this {
+extend list {
     function flatten() {
         return flatten(this);
     }

modules/collection/src/main/resources/map.mus

@@ -1,4 +1,4 @@
-extend map as this {
+extend map {
     function containsKey(key) {
         return this.keys.contains(key);
     }

modules/music/src/main/resources/main.mus

@@ -1,4 +1,4 @@
-extend note as this {
+extend note {
     function withOctave(octave: int) {
         return Note(this.pitch, octave, this.duration.get("numerator"), this.duration.get("denominator"));
     }

modules/text/src/main/resources/main.mus

@@ -1,4 +1,4 @@
-extend string as this {
+extend string {
     function find(char: string) {
         if(char.length > 1) {
             throw "Only single character can act as a pattern to be found";