icons: Let the system using the icons specify the fill color.

The fill color is ignored when we generate webfonts in our main app.
For our help center beta project using Astro, we would have needed to
come up with a way to change fill colors for all the icons. Since, there
was no reason for us to keep the fill colors in the SVGs, we decided to
remove it instead and add a lint rule to keep that in check.
See
https://chat.zulip.org/#narrow/channel/6-frontend/topic/fill.20in.20our.20current.20svg.20icons
for more details.

tools/linter_lib/custom_check.py

@@ -966,6 +966,18 @@ txt_rules = RuleList(
     langs=["txt", "text", "yaml", "yml"],
     rules=whitespace_rules,
 )
+
+svg_rules = RuleList(
+    langs=["svg"],
+    rules=[
+        {
+            "pattern": r"fill=(['\"])(.*?)\1",
+            "description": "System icons ignore fill values, so do not include the fill property.",
+            "include_only": {"web/shared/icons/", "web/images/icons/"},
+        },
+    ],
+)
+
 non_py_rules = [
     handlebars_rules,
     jinja2_rules,
@@ -978,4 +990,5 @@ non_py_rules = [
     txt_rules,
     puppet_rules,
     openapi_rules,
+    svg_rules,
 ]