linter: Fix parsing features.txt

Previously, split() would split on whitespace, then the if-condition would
remove `#` - interpreting every word in every comment in the file as a
potential valid feature flag. We want splitlines() here.

Partial-line comments were inadvertently "supported" before, because of
this bug. Instead, support them explicitly by chopping off a `#`
character, anything after it, and any whitespace immediately preceding it.

tools/lint/lib/checks/features.py

@@ -1,3 +1,5 @@
+import re
+
 from ..check import Check
 
 class CheckFeatures(Check):
@@ -11,8 +13,9 @@ class CheckFeatures(Check):
     @staticmethod
     def _parse(content):
         features = []
-        for line in content.split():
-            if not line or line.startswith('#'):
+        for line in content.splitlines():
+            line = re.sub(r'\s*#.*', '', line)
+            if not line:
                 continue
             features.append(line)
         return features

tools/lint/test/fixtures/features.txt

@@ -1,4 +1,5 @@
-async-functions
+# Here is a full-line comment: not-a-valid-feature
+async-functions  # This is a partial-line comment
 object-spread
 generators
 BigInt