Move IsReadmeFile* from modules/markup/ to modules/util (#22877)

These functions don't examine contents, just filenames, so they don't
fit in well in a markup module.

This was originally part of
https://github.com/go-gitea/gitea/pull/22177.

Signed-off-by: Nick Guenther <nick.guenther@polymtl.ca>

modules/util/path_test.go

@@ -55,3 +55,84 @@ func TestFileURLToPath(t *testing.T) {
 		}
 	}
 }
+
+func TestMisc_IsReadmeFileName(t *testing.T) {
+	trueTestCases := []string{
+		"readme",
+		"README",
+		"readME.mdown",
+		"README.md",
+		"readme.i18n.md",
+	}
+	falseTestCases := []string{
+		"test.md",
+		"wow.MARKDOWN",
+		"LOL.mDoWn",
+		"test",
+		"abcdefg",
+		"abcdefghijklmnopqrstuvwxyz",
+		"test.md.test",
+		"readmf",
+	}
+
+	for _, testCase := range trueTestCases {
+		assert.True(t, IsReadmeFileName(testCase))
+	}
+	for _, testCase := range falseTestCases {
+		assert.False(t, IsReadmeFileName(testCase))
+	}
+
+	type extensionTestcase struct {
+		name     string
+		expected bool
+		idx      int
+	}
+
+	exts := []string{".md", ".txt", ""}
+	testCasesExtensions := []extensionTestcase{
+		{
+			name:     "readme",
+			expected: true,
+			idx:      2,
+		},
+		{
+			name:     "readme.md",
+			expected: true,
+			idx:      0,
+		},
+		{
+			name:     "README.md",
+			expected: true,
+			idx:      0,
+		},
+		{
+			name:     "ReAdMe.Md",
+			expected: true,
+			idx:      0,
+		},
+		{
+			name:     "readme.txt",
+			expected: true,
+			idx:      1,
+		},
+		{
+			name:     "readme.doc",
+			expected: true,
+			idx:      3,
+		},
+		{
+			name: "readmee.md",
+		},
+		{
+			name:     "readme..",
+			expected: true,
+			idx:      3,
+		},
+	}
+
+	for _, testCase := range testCasesExtensions {
+		idx, ok := IsReadmeFileExtension(testCase.name, exts...)
+		assert.Equal(t, testCase.expected, ok)
+		assert.Equal(t, testCase.idx, idx)
+	}
+}