Use AlternateLookup with ReadOnlySpan<char>

[russcam]

Lingua performs lots of lookups into Dictionary<string, double> to get n-gram probabilities. In net9.0, use the new .GetAlternateLookup<T>() API to lookup using a ReadOnlySpan<char>.

Benchmarking shows a nice perf gain

Before

BenchmarkDotNet v0.14.0, Windows 11 (10.0.22631.4391/23H2/2023Update/SunValley3)
13th Gen Intel Core i9-13900K, 1 CPU, 32 logical and 24 physical cores
.NET SDK 9.0.100
  [Host]   : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2
  ShortRun : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2

Job=ShortRun  IterationCount=3  LaunchCount=1
WarmupCount=3

Method	Text	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
LinguaLowAccuracy	On n(...)own. [125]	79.22 us	16.115 us	0.883 us	1.00	0.01	7.3242	-	-	136.42 KB	1.00
Lingua	On n(...)own. [125]	77.41 us	2.903 us	0.159 us	0.98	0.01	7.3242	-	-	136.42 KB	1.00
LanguageDetection	On n(...)own. [125]	108.66 us	40.258 us	2.207 us	1.37	0.03	2.6855	2.3193	1.3428	260.51 KB	1.91
NTextCat	On n(...)own. [125]	170.72 us	7.196 us	0.394 us	2.16	0.02	6.1035	0.7324	-	116.13 KB	0.85

After

BenchmarkDotNet v0.14.0, Windows 11 (10.0.22631.4391/23H2/2023Update/SunValley3)
13th Gen Intel Core i9-13900K, 1 CPU, 32 logical and 24 physical cores
.NET SDK 9.0.100
  [Host]   : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2
  ShortRun : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2

Job=ShortRun  IterationCount=3  LaunchCount=1
WarmupCount=3

Method	Text	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
LinguaLowAccuracy	On n(...)own. [125]	67.25 us	9.423 us	0.516 us	1.00	0.01	6.2256	-	-	115.33 KB	1.00
Lingua	On n(...)own. [125]	66.32 us	3.677 us	0.202 us	0.99	0.01	6.2256	-	-	115.33 KB	1.00
LanguageDetection	On n(...)own. [125]	112.16 us	22.231 us	1.219 us	1.67	0.02	2.6855	2.3193	1.3428	260.48 KB	2.26
NTextCat	On n(...)own. [125]	172.06 us	23.023 us	1.262 us	2.56	0.02	6.1035	0.7324	-	116.13 KB	1.01