Unterminated Raku heredoc produces a panic

[JRaspass]

Describe the bug

An unterminated heredoc in Raku causes chroma to panic.

To Reproduce

--- a/lexers/testdata/raku.actual
+++ b/lexers/testdata/raku.actual
@@ -495,6 +495,8 @@ say Calculator.parse('2 + 3', actions => Calculations).made;
 
 put Date.today.later(:2years).year;
 
+say q:to/Foo/;
+
 =finish
 C<say> Date.today.year;
 # Output: 2020

$ go test -run ^TestLexers$ ./lexers
--- FAIL: TestLexers (13.90s)
    --- FAIL: TestLexers/Raku/testdata/raku.actual (4.86s)
panic: runtime error: slice bounds out of range [12136:12133] [recovered]
	panic: runtime error: slice bounds out of range [12136:12133]

goroutine 177 [running]:
testing.tRunner.func1.2({0x7934a0, 0xc0017bac30})
	/usr/lib/go/src/testing/testing.go:1209 +0x24e
testing.tRunner.func1()
	/usr/lib/go/src/testing/testing.go:1212 +0x218
panic({0x7934a0, 0xc0017bac30})
	/usr/lib/go/src/runtime/panic.go:1038 +0x215
github.com/alecthomas/chroma/lexers/r.rakuRules.func1.1(0xc001784210)
	/home/jraspass/chroma/lexers/r/raku.go:541 +0x1145
github.com/alecthomas/chroma.MutatorFunc.Mutate(0xc00132e950, 0x2fc0)
	/home/jraspass/chroma/mutators.go:24 +0x1f
github.com/alecthomas/chroma.(*LexerState).Iterator(0xc001784210)
	/home/jraspass/chroma/regexp.go:363 +0x3a2
github.com/alecthomas/chroma.(*coalescer).Tokenise.func1()
	/home/jraspass/chroma/coalesce.go:15 +0x2f
github.com/alecthomas/chroma.Tokenise({0x8e5188, 0xc000f4db60}, 0x3044, {0xc001254000, 0x0})
	/home/jraspass/chroma/regexp.go:191 +0x91
github.com/alecthomas/chroma/lexers_test.FileTest.func1(0x0)
	/home/jraspass/chroma/lexers/lexers_test.go:54 +0xdf
testing.tRunner(0xc001791520, 0xc0014e6600)
	/usr/lib/go/src/testing/testing.go:1259 +0x102
created by testing.(*T).Run
	/usr/lib/go/src/testing/testing.go:1306 +0x35a
FAIL	github.com/alecthomas/chroma/lexers	13.907s
FAIL

[alecthomas]

@CIAvash any ideas?

[CIAvash]

I will look into it when I get the time.