Feat: update VSCode extension with compound assignment and shift operators

Add compound assignment operators (+=, -=, *=, /=, %=, &=, |=, ^=,
<<=, >>=) and shift operators (<<, >>) to the tmLanguage syntax
highlighting grammar. Patterns are ordered longest-first to prevent
shorter tokens from shadowing multi-character operators. Also update
fibonacci example to use += compound assignment.

vscode-flux/syntaxes/flux.tmLanguage.json

@@ -156,19 +156,29 @@
     "operators": {
       "patterns": [
         {
-          "comment": "Arrow (return type separator)",
+          "comment": "Arrow (return type separator) — must precede - and -=",
           "name": "keyword.operator.arrow.flux",
           "match": "->"
         },
+        {
+          "comment": "Compound assignment operators — must precede shift, comparison, and arithmetic",
+          "name": "keyword.operator.assignment.compound.flux",
+          "match": "<<=|>>=|\\+=|-=|\\*=|/=|%=|&=|\\|=|\\^="
+        },
+        {
+          "comment": "Shift operators — must precede < and >",
+          "name": "keyword.operator.bitwise.shift.flux",
+          "match": "<<|>>"
+        },
         {
           "comment": "Comparison operators",
           "name": "keyword.operator.comparison.flux",
           "match": "==|!=|<=|>=|<|>"
         },
         {
-          "comment": "Assignment",
+          "comment": "Assignment — negative lookbehind excludes all compound-assign prefixes",
           "name": "keyword.operator.assignment.flux",
-          "match": "(?<![=!<>])=(?!=)"
+          "match": "(?<![=!<>+\\-*/%&|^])=(?!=)"
         },
         {
           "comment": "Arithmetic: + - * / %",
@@ -176,7 +186,7 @@
           "match": "[+\\-*/%]"
         },
         {
-          "comment": "Bitwise: & | ^ ~ and unary ! ~",
+          "comment": "Bitwise and unary: & | ^ ~ !",
           "name": "keyword.operator.bitwise.flux",
           "match": "[&|^~!]"
         }