Fix multibyte character tokenization bug in ERB::Util

[martinemde]

Motivation / Background

This Pull Request has been created because there are problems parsing certain ERB templates that cause error highlighting to be wrong.

Detail

StringScanner uses bytes, so use byteslice so that all length calculations are done in bytes.

Additional information

The tests I added fail as follows when multibyte characters exist before the tag:

Failure:
ActiveSupport::ERBUtilTest#test_multibyte_characters_start [test/core_ext/erb_util_test.rb:141]:
--- expected
+++ actual
@@ -1 +1 @@
-[[:TEXT, "こんにちは"], [:OPEN, "<%="], [:CODE, " name "], [:CLOSE, "%>"]]
+[[:TEXT, "こんにちは<%= name %"], [:OPEN, "<%="], [:CODE, " name "], [:CLOSE, "%>"]]

Failure:
ActiveSupport::ERBUtilTest#test_multibyte_characters_end [test/core_ext/erb_util_test.rb:151]:
--- expected
+++ actual
@@ -1 +1 @@
-[[:CODE, " 'こんにちは' "], [:CLOSE, "%>"]]
+[[:CODE, " 'こんにちは' %>"], [:CLOSE, "%>"]]

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.