parser: Fix return type of Buffer#line_range and #source_range (#1017)

The return type of Parser::Source::Buffer#line_range and #source_range
are `Parser::Source::Range` from parser gem, not built-in `Range`.

This is a verification script to confirm the return type of them:

```ruby
require 'parser/ruby34'

source = "hello\nworld\n"
buffer = Parser::Source::Buffer.new('(test)', source: source)

line_range = buffer.line_range(1)
p line_range.class

source_range = buffer.source_range
p source_range.class
```

ref: https://github.com/whitequark/parser/blob/v3.2.2.4/lib/parser/source/buffer.rb#L291

Author: tk0miya

gems/parser/3.2/parser.rbs

@@ -113,8 +113,8 @@ module Parser
       def decompose_position: (Integer) -> [Integer, Integer]
       def source_lines: () -> Array[String]
       def source_line: (Integer) -> String
-      def line_range: (Integer) -> ::Range[Integer]
-      def source_range: () -> ::Range[Integer]
+      def line_range: (Integer) -> Range
+      def source_range: () -> Range
       def last_line: () -> Integer
     end