Improve PowerShell escaping fixtures, refine regexes

Expand test fixtures for PowerShell escaping based on mutation testing
results. This mostly identified gaps in the testing where the use of the
"g" flag wasn't covered by tests due to a lack of repeated characters to
escape in a string.

Additionally, based on mutation testing too, update some of the regular
expressions. Expressions anchored to the start or end of the string
don't need to have the "g" flag and the `backslashSuffix` regex can't
possibly match the start of the string because it requires at least a
non-whitespace character at the start and a whitespace character after
that (because of the guard in which this expression is used).

Author: ericcornelissen

src/internal/win/powershell.js

@@ -20,10 +20,10 @@ export function getEscapeFunction() {
   const specials2 = new RegExp("([$&'(),;{|}‘’‚‛“”„])", "g");
 
   const whitespace = new RegExp("([\\s\u0085])", "g");
-  const whitespacePrefix = new RegExp("^[\\s\u0085]+", "g");
+  const whitespacePrefix = new RegExp("^[\\s\u0085]+");
 
   const quote = new RegExp('(^|[^\\\\])(\\\\*)"', "g");
-  const backslashSuffix = new RegExp("(^|[^\\\\])(\\\\+)$", "g");
+  const backslashSuffix = new RegExp("([^\\\\])(\\\\+)$");
 
   return (arg) => {
     arg = arg

test/fixtures/win.js

@@ -3074,6 +3074,10 @@ export const escape = {
         input: "a\u0085@b",
         expected: "a`\u0085`@b",
       },
+      {
+        input: "a @b @c",
+        expected: "a` `@b` `@c",
+      },
     ],
     "hashtags ('#')": [
       {
@@ -3110,6 +3114,10 @@ export const escape = {
         input: "a\u0085#b",
         expected: "a`\u0085`#b",
       },
+      {
+        input: "a #b #c",
+        expected: "a` `#b` `#c",
+      },
     ],
     "carets ('^')": [
       {
@@ -3218,6 +3226,10 @@ export const escape = {
         input: "a\u0085-b",
         expected: "a`\u0085`-b",
       },
+      {
+        input: "a -b -c",
+        expected: "a` `-b` `-c",
+      },
     ],
     "backslashes ('\\')": [
       {
@@ -3236,6 +3248,10 @@ export const escape = {
         input: "a\\",
         expected: "a\\",
       },
+      {
+        input: "\\",
+        expected: "\\",
+      },
     ],
     "backslashes ('\\') + whitespace": [
       {
@@ -3274,6 +3290,14 @@ export const escape = {
         input: "  a b\\",
         expected: "` ` a` b\\\\",
       },
+      {
+        input: " \\",
+        expected: "` \\",
+      },
+      {
+        input: "\\ \\",
+        expected: "\\` \\\\",
+      },
     ],
     "colons (':')": [
       {
@@ -3310,6 +3334,10 @@ export const escape = {
         input: "a\u0085:b",
         expected: "a`\u0085`:b",
       },
+      {
+        input: "a :b :c",
+        expected: "a` `:b` `:c",
+      },
     ],
     "semicolons (';')": [
       {
@@ -3466,6 +3494,18 @@ export const escape = {
         input: "a\u0085]b",
         expected: "a`\u0085`]b",
       },
+      {
+        input: "a [b [c",
+        expected: "a` [b` [c",
+      },
+      {
+        input: "a ]b ]c",
+        expected: "a` `]b` `]c",
+      },
+      {
+        input: "a [b ]c",
+        expected: "a` [b` `]c",
+      },
     ],
     "curly brackets ('{', '}')": [
       {
@@ -3568,6 +3608,14 @@ export const escape = {
         input: "a\u0085>b",
         expected: "a`\u0085`>b",
       },
+      {
+        input: "a <b <c",
+        expected: "a` `<b` `<c",
+      },
+      {
+        input: "a >b >c",
+        expected: "a` `>b` `>c",
+      },
     ],
     "right angle brackets ('>') + digits (/[0-9]/)": [
       {
@@ -3650,6 +3698,10 @@ export const escape = {
         input: "a 0>b",
         expected: "a` 0>b",
       },
+      {
+        input: "a 0>b 1>c 2>d 3>e 4>f 5>g 6>h 7>i 8>j 9>k",
+        expected: "a` 0>b` 1`>c` 2`>d` 3`>e` 4`>f` 5`>g` 6`>h` 7>i` 8>j` 9>k",
+      },
       {
         input: "a\t1>b",
         expected: "a`\t1`>b",
@@ -3748,6 +3800,10 @@ export const escape = {
         input: "a\u0085*>b",
         expected: "a`\u0085*`>b",
       },
+      {
+        input: "a *>b *>c",
+        expected: "a` *`>b` *`>c",
+      },
     ],
     "left double quotation mark ('“')": [
       {