fix: make enum lookup case insensitive

[joshgummersall]

I tested the HTTP action in Composer with the .NET runtime, and it appears that enum resolution is case insensitive, so this PR replicates that behavior. I tested this by linking against a Composer bot and manually editing the enum value in the dialog to different casing variations.

Fixes https://github.com/microsoft/BotFramework-Composer/issues/7834
Fixes #3744

[coveralls]

Pull Request Test Coverage Report for Build 937966284

• 9 of 10 (90.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.008%) to 85.406%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/adaptive-expressions/src/converters/enumExpressionConverter.ts	9	10	90.0%

Files with Coverage Reduction	New Missed Lines	%
libraries/adaptive-expressions/src/converters/enumExpressionConverter.ts	1	73.08%
libraries/botbuilder-lg/src/templateExtensions.ts	1	84.48%

Totals
Change from base Build 937811099:	-0.008%
Covered Lines:	19273
Relevant Lines:	21467

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💛 - Coveralls

[joshgummersall]

@stevengum, as far as I can tell, Dotnet explicitly sets case insensitive here: https://github.com/microsoft/botbuilder-dotnet/blob/main/libraries/AdaptiveExpressions/ExpressionProperties/EnumExpression.cs#L104

[stevengum]

One small ask and a suggestion for cleanup. Otherwise LGTM