Add upper-case I as a possible space replacement fallback in Font.spaceWidth to improve text-selection (issue 7180)

In fonts with only upper-case glyphs, that are also missing a space glyph, `get spaceWidth` won't be able to return anything useful.
By adding upper-case `I` as a fallback, we can thus improve text-selection in some PDF files.
Note that locally, the patch causes slight movement in a few existing `text` tests, but in my opinion this actually looks like slight improvements.

Fixes 7180.

test/pdfs/.gitignore

@@ -24,6 +24,7 @@
 !issue7020.pdf
 !issue7101.pdf
 !issue7115.pdf
+!issue7180.pdf
 !issue7200.pdf
 !issue7229.pdf
 !filled-background.pdf

test/test_manifest.json

@@ -1555,6 +1555,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue7180-text",
+       "file": "pdfs/issue7180.pdf",
+       "md5": "73ed92d7ca55475f1f31d1d75fee3283",
+       "rounds": 1,
+       "link": false,
+       "type": "text"
+    },
     {
       "id": "bug1123803",
       "file": "pdfs/bug1123803.pdf",