Make loop unrolling completely whitespace-insensitive

This is just another #19726.
Since #20012 is accepted, I think it's okay to make loop unrolling
completely whitespace-insensitive.

docs/api/en/materials/ShaderMaterial.html

@@ -54,9 +54,6 @@ <h1>[name]</h1>
 					<li>
 						The loop variable has to be *i*.
 					</li>
-					<li>
-						The loop has to use a certain whitespace formatting.
-					</li>
 				</ul>
 				<code>
 		#pragma unroll_loop_start

docs/api/zh/materials/ShaderMaterial.html

@@ -43,9 +43,6 @@ <h1>着色器材质([name])</h1>
 					<li>
 						循环变量必须是*i*。
 					</li>
-					<li>
-						循环必须使用某种空格格式。
-					</li>
 				</ul>
 				<code>
 		#pragma unroll_loop_start

src/renderers/webgl/WebGLProgram.js

@@ -220,7 +220,7 @@ function includeReplacer( match, include ) {
 // Unroll Loops
 
 const deprecatedUnrollLoopPattern = /#pragma unroll_loop[\s]+?for \( int i \= (\d+)\; i < (\d+)\; i \+\+ \) \{([\s\S]+?)(?=\})\}/g;
-const unrollLoopPattern = /#pragma unroll_loop_start[\s]+?for\s*\(\s*int i \= (\d+)\; i < (\d+)\; i\s*\+\+\s*\)\s*{([\s\S]+?)(?=\})\}[\s]+?#pragma unroll_loop_end/g;
+const unrollLoopPattern = /#pragma\s+unroll_loop_start\s+for\s*\(\s*int\s+i\s*=\s*(\d+)\s*;\s*i\s*<\s*(\d+)\s*;\s*i\s*\+\+\s*\)\s*{([\s\S]+?)}\s+#pragma\s+unroll_loop_end/g;
 
 function unrollLoops( string ) {
 
@@ -244,7 +244,7 @@ function loopReplacer( match, start, end, snippet ) {
 	for ( let i = parseInt( start ); i < parseInt( end ); i ++ ) {
 
 		string += snippet
-			.replace( /\[ i \]/g, '[ ' + i + ' ]' )
+			.replace( /\[\s*i\s*\]/g, '[ ' + i + ' ]' )
 			.replace( /UNROLLED_LOOP_INDEX/g, i );
 
 	}