Fix Windows path handling for nested jars

Update `Path` creation for nested locations to allow both UNC and classic
file references to be used. This commit attempts to align our URL
handling with that of standard file URLs. The `NestedLocation` class
no longer attempts to remove leading all `\` characters and instead
only removes the first `\` when the second char is `:`. This duplicates
the logic found in Java's own internal `WindowsUriSupport` class which
is used when calling `Path.of(url)` with a `file:` URL.

Fixes gh-40549

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/net/protocol/nested/NestedLocationTests.java

@@ -85,7 +85,7 @@ class NestedLocationTests {
 	@Test
 	void fromUrlWhenNoPathThrowsException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> NestedLocation.fromUrl(new URL("nested:")))
-			.withMessageContaining("'path' must not be empty");
+			.withMessageContaining("'location' must not be empty");
 	}
 
 	@Test