Run CI with --enable-frozen-string-literal

Ensure the gem is compatible with it.

The only issues are in `parser.rb`, that's the only file that
doesn't enable frozen strings. I'm not sure if it was done on
purpose or not.

.github/workflows/ci.yml

@@ -10,7 +10,11 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu]
-        ruby: [ head, 3.2 ]
+        ruby: [ head, '3.2', '3.3' ]
+        ruby-opt: [""]
+        include:
+          - ruby: "3.3"
+            ruby-opt: "--enable-frozen-string-literal --debug-frozen-string-literal"
 
     steps:
       - name: Install Protoc
@@ -24,5 +28,5 @@ jobs:
 
       - name: Install dependencies
         run: bundle install
-      - name: Run tests
-        run: bundle exec rake
+      - name: Run tests ${{ matrix.rubyopt }}
+        run: bundle exec rake RUBYOPT="${{ matrix.rubyopt }}"

lib/protoboeuf/parser.rb

@@ -719,7 +719,7 @@ def eat_ws()
     # bar_bif
     # foo123
     def read_ident
-      name = ''
+      name = +''
 
       loop do
         if eof?
@@ -745,7 +745,7 @@ def read_ident
     # Read a string constant, eg:
     # 'foobar'
     def read_string
-      str = ''
+      str = +''
 
       # The string must start with an opening quote
       expect '"'