Fix the charCodeOf method in IdentityToUnicodeMap in order to prevent text selection from breaking

After PR 6590, `font.spaceWidth` is now called in more cases than before (in `PartialEvaluator_getTextContent`), which exposed an underlying issue with `IdentityToUnicodeMap_charCodeOf` throwing an error.
This breaks text-selection in some PDF files found in the wild, hence this patch replaces the `error` with an actual function instead (modelled after `IdentityCMap_charCodeOf`).

test/pdfs/.gitignore

@@ -179,6 +179,7 @@
 !issue5549.pdf
 !issue5475.pdf
 !annotation-border-styles.pdf
+!IdentityToUnicodeMap_charCodeOf.pdf
 !issue5481.pdf
 !issue5567.pdf
 !issue5701.pdf

test/pdfs/IdentityToUnicodeMap_charCodeOf.pdf

@@ -0,0 +1,68 @@
+%PDF-1.7
+%âãÏÓ
+1 0 obj 
+<<
+/Pages 2 0 R
+/Type /Catalog
+>>
+endobj 
+2 0 obj 
+<<
+/Kids [3 0 R]
+/Count 1
+/Type /Pages
+>>
+endobj 
+3 0 obj 
+<<
+/Parent 2 0 R
+/MediaBox [0 0 200 50]
+/Resources 
+<<
+/Font 
+<<
+/F1 4 0 R
+>>
+>>
+/Contents 5 0 R
+/Type /Page
+>>
+endobj 
+4 0 obj 
+<<
+/BaseFont /Times-Roman
+/Subtype /Type1
+/ToUnicode /Identity-H
+/Encoding /WinAnsiEncoding
+/Type /Font
+>>
+endobj 
+5 0 obj 
+<<
+/Length 37
+>>
+stream
+BT
+10 20 TD
+/F1 20 Tf
+(ABCdef) Tj
+ET
+
+endstream 
+endobj xref
+0 6
+0000000000 65535 f 
+0000000015 00000 n 
+0000000066 00000 n 
+0000000125 00000 n 
+0000000254 00000 n 
+0000000378 00000 n 
+trailer
+
+<<
+/Root 1 0 R
+/Size 6
+>>
+startxref
+467
+%%EOF

test/test_manifest.json

@@ -1453,6 +1453,13 @@
       "lastPage": 1,
       "type": "eq"
     },
+    {  "id": "IdentityToUnicodeMap_charCodeOf",
+       "file": "pdfs/IdentityToUnicodeMap_charCodeOf.pdf",
+       "md5": "da030686418c5e37d889127a05dafb83",
+       "rounds": 1,
+       "link": false,
+       "type": "text"
+    },
     {  "id": "bug894572",
        "file": "pdfs/bug894572.pdf",
        "md5": "e54a6b0451939f685ed37e3d46e16158",