Fix indentation preservation in JSDoc (microsoft#37717)

This fixes two bugs in the parseJSDocCommentWorker().

1. The initial indent was calculated wrongly. It was set to the
   difference between the index of the last newline or beginning of file
   and the current start marker (position of /**). By calculating it
   this way, the newline character itself is counted as indentation
   character as well. The initial indent is used as margin for the
   whole comment. The margin contains the amount of characters to skip
   before the actual content or payload of a comment line. The algorithm
   does not skip non-whitespace characters at the beginning of the
   content, but it would strip away one whitespace character for
   indented content (which does matter, if there is e.g. a Markdown
   code block with indentation in the comment).

2. When reducing initial whitespace sequences of comment lines by the
   remaining margin the algorithm cut off one character too much. This
   might have been introduced to fix 1. It had a similar effect as 1.

Author: Neonit

src/compiler/parser.ts

@@ -6884,7 +6884,8 @@ namespace ts {
                     let state = JSDocState.SawAsterisk;
                     let margin: number | undefined;
                     // + 4 for leading '/** '
-                    let indent = start - Math.max(content.lastIndexOf("\n", start), 0) + 4;
+                    // + 1 because the last index of \n is always one index before the first character in the line and coincidentally, if there is no \n before start, it is -1, which is also one index before the first character
+                    let indent = start - (content.lastIndexOf("\n", start) + 1) + 4;
                     function pushComment(text: string) {
                         if (!margin) {
                             margin = indent;
@@ -6940,7 +6941,7 @@ namespace ts {
                                     comments.push(whitespace);
                                 }
                                 else if (margin !== undefined && indent + whitespace.length > margin) {
-                                    comments.push(whitespace.slice(margin - indent - 1));
+                                    comments.push(whitespace.slice(margin - indent));
                                 }
                                 indent += whitespace.length;
                                 break;

tests/cases/fourslash/jsDocIndentationPreservation1.ts

@@ -0,0 +1,13 @@
+///<reference path="fourslash.ts" />
+// @allowJs: true
+// @Filename: Foo.js
+
+/////**
+//// * Does some stuff.
+//// *     Second line.
+//// * 	Third line.
+//// */
+////function foo/**/(){}
+
+goTo.marker();
+verify.quickInfoIs("function foo(): void", "Does some stuff.\n    Second line.\n\tThird line.");

tests/cases/fourslash/jsDocIndentationPreservation2.ts

@@ -0,0 +1,13 @@
+///<reference path="fourslash.ts" />
+// @allowJs: true
+// @Filename: Foo.js
+
+/////**
+////	Does some stuff.
+////	    Second line.
+////		Third line.
+////*/
+////function foo/**/(){}
+
+goTo.marker();
+verify.quickInfoIs("function foo(): void", "Does some stuff.\n    Second line.\n\tThird line.");

tests/cases/fourslash/jsDocIndentationPreservation3.ts

@@ -0,0 +1,13 @@
+///<reference path="fourslash.ts" />
+// @allowJs: true
+// @Filename: Foo.js
+
+/////**
+////    Does some stuff.
+////        Second line.
+////    	Third line.
+////*/
+////function foo/**/(){}
+
+goTo.marker();
+verify.quickInfoIs("function foo(): void", "Does some stuff.\n    Second line.\n\tThird line.");