Percent encode the URI template literals as per RFC6570 section 3.1

Template::expandAll swapped every expression notation in the raw template
string with str_replace, so the literal text surrounding the expressions was
copied verbatim and never percent encoded. The expression detection pattern is
now used to split the template into an ordered list of parts: literals are
encoded once at construction, expressions stay Expression objects, and the
expansion simply concatenates them. Keeping the parts apart is what prevents
the already encoded expansion results from being encoded a second time.

Fixes the "Additional Examples 8: Literal Encoding" case of the vendored
uritemplate-test compliance suite.

uri/UriTemplate/TemplateTest.php

@@ -174,6 +174,63 @@ final class TemplateTest extends TestCase
         ];
     }
 
+    #[DataProvider('providesLiteralEncoding')]
+    #[Test]
+    public function testItEncodesTheTemplateLiterals(string $notation, array $variables, string $expected): void
+    {
+        self::assertSame($expected, Template::new($notation)->expand($variables));
+    }
+
+    /**
+     * @see https://www.rfc-editor.org/rfc/rfc6570#section-3.1
+     */
+    public static function providesLiteralEncoding(): iterable
+    {
+        $variables = [
+            'var' => 'value',
+            'path' => '/foo/bar',
+            'hello' => 'Hello World!',
+        ];
+
+        return [
+            'a character disallowed in a URI is encoded as its UTF-8 octets' => [
+                'notation' => 'café/{var}',
+                'variables' => $variables,
+                'expected' => 'caf%C3%A9/value',
+            ],
+            'a percent encoded triplet is copied as is' => [
+                'notation' => 'x%20y/{var}',
+                'variables' => $variables,
+                'expected' => 'x%20y/value',
+            ],
+            'a percent encoded triplet is copied as is on both sides of an expression' => [
+                'notation' => 'x%20y{var}z%20w',
+                'variables' => $variables,
+                'expected' => 'x%20yvaluez%20w',
+            ],
+            'a space is encoded' => [
+                'notation' => 'a b/{var}',
+                'variables' => $variables,
+                'expected' => 'a%20b/value',
+            ],
+            'a percent sign which starts no triplet is encoded' => [
+                'notation' => '100%/{var}',
+                'variables' => $variables,
+                'expected' => '100%25/value',
+            ],
+            'reserved characters are copied as is' => [
+                'notation' => "/a[0]:b@c!d\$e&f'g(h)i*j+k,l;m=n?o#{var}",
+                'variables' => $variables,
+                'expected' => "/a[0]:b@c!d\$e&f'g(h)i*j+k,l;m=n?o#value",
+            ],
+            'the expanded expressions are never encoded twice' => [
+                'notation' => 'café{+path}{#hello}',
+                'variables' => $variables,
+                'expected' => 'caf%C3%A9/foo/bar#Hello%20World!',
+            ],
+        ];
+    }
+
     public function testExpandOrFailIfAtLeastOneVariableIsMissing(): void
     {
         $this->expectException(TemplateCanNotBeExpanded::class);