script: Fix panic in htmlimageelement.rs using str::find() to find character boundaries. (#32980)

* fix loop with chars().enumerate() by using find()

Signed-off-by: Kopanov Anton <anton.kopanov@ya.ru>

* Add documentation to parser and fix some small issues

- Rename the properties of `Descriptor` so that they are full words
- Use the Rust-parser to parse doubles
- Add documentation and restructure parser to be more readable

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

---------

Signed-off-by: Kopanov Anton <anton.kopanov@ya.ru>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/meta/html/semantics/embedded-content/the-img-element/sizes/sizes-auto-rendering-2.html.ini

@@ -1,2 +0,0 @@
-[sizes-auto-rendering-2.html]
-  expected: FAIL

tests/wpt/meta/html/semantics/embedded-content/the-img-element/sizes/sizes-auto-rendering-3.html.ini

@@ -0,0 +1,2 @@
+[sizes-auto-rendering-3.html]
+  expected: FAIL

tests/wpt/meta/html/semantics/embedded-content/the-img-element/sizes/sizes-auto-rendering.html.ini

@@ -0,0 +1,2 @@
+[sizes-auto-rendering.html]
+  expected: FAIL

tests/wpt/meta/html/semantics/embedded-content/the-img-element/update-the-image-data/fail-to-resolve.html.ini

@@ -1,3 +0,0 @@
-[fail-to-resolve.html]
-  [<img srcset="//[, /images/red.png">]
-    expected: FAIL

tests/wpt/meta/html/semantics/embedded-content/the-img-element/update-the-source-set.html.ini

@@ -1,7 +1,4 @@
 [update-the-source-set.html]
-  [<img src="data:,a" srcset="data:,b 1e0x" data-expect="data:,b">]
-    expected: FAIL
-
   [<picture><source srcset="data:,b" type=""><img src="data:,a" data-expect="data:,b"></picture>]
     expected: FAIL