Replace stripJsonComments() trailing comma regexp

I realized after merging #11 that the regular expression at the end of
stripJsonComments() would affect quoted strings. The new
"stripJsonComments > doesn't modify > strings including trailing commas"
test case confirmed this.

The new algorithm took a bit of trial and error, and the existing tests
really helped distill it to its essence. Quite happy with how it turned
out.

Also reorganized the test cases with more `describe` blocks. Added
the new "maintains correct syntax error position info when..." suite to
confirm positional info in JSON.parse errors remains correct.

lib/index.js

@@ -185,14 +185,12 @@ export async function analyzeArgv(argv) {
  * @returns {string} jsonStr with comments, trailing commas replaced by space
  */
 export function stripJsonComments(jsonStr) {
-  let inString = false
-  let escaped = false
-  let inComment = null
-  let result = ''
+  let inString, escaped, inComment, comma, result = []
 
   for (let i = 0; i !== jsonStr.length; ++i) {
-    const prevChar = i !== 0 ? jsonStr[i-1] : null
+    const prevChar = jsonStr[i-1]
     let curChar = jsonStr[i]
+    const isNotWhitespace = curChar.trimStart() !== ''
 
     if (inString) {
       inString = curChar !== '"' || escaped
@@ -202,19 +200,22 @@ export function stripJsonComments(jsonStr) {
           (inComment === 'block' && prevChar === '*' && curChar === '/')) {
         inComment = null
       }
-      if (curChar.trimStart() !== '') curChar = ' '
-    } else if (curChar === '"') {
+      if (isNotWhitespace) curChar = ' '
+    } else if (curChar === '"') {  // opening a string
       inString = true
-    } else if (prevChar === '/') {
-      if (curChar === '/') inComment = 'line'
-      else if (curChar === '*') inComment = 'block'
-      if (inComment) curChar = ' '  // otherwise prevChar closed a block comment
-    } else if (curChar === '/') {
-      curChar = ' '  // opening a line or block comment
+      comma = null
+    } else if (curChar === '/' || curChar === '*') {  // maybe opening a comment
+      if (prevChar === '/') {  // definitely opening a comment
+        inComment = (curChar === '/') ? 'line' : 'block'
+        curChar = result[i-1] = ' '
+      }
+    } else if (isNotWhitespace) {  // definitely outside string or comment
+      if (comma && (curChar === ']' || curChar === '}')) result[comma] = ' '
+      comma = (curChar === ',') ? i : null
     }
-    result += curChar
+    result.push(curChar)
   }
-  return result.replaceAll(/,(\s*[\]}])/g, ' $1')
+  return result.join('')
 }
 
 /**