Added assertion up to cv99 for FontFeature.characterVariant

[Darkade]

This PR changes the assertion in factory FontFeature.characterVaraint from up to 20 to up to 99 in order to support additional Character Variants

• Previously whenever you would setup to use a character variant after 20 flutter would give an error during runtime.

You can check the OpenType docs to confirm the character variants go from cv01 to cv99

https://docs.microsoft.com/en-us/typography/opentype/spec/features_ae#cv01-cv99

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[pazthor]

I like to expand it to what the specification indicates; my only issue would be what it mentions about glyphs :

the number of variants should be identical for all glyphs.

What do you think about that @Darkade?

[aalkz]

This is great, it would allow OTF fonts with specific variants to be super useful and would also allow the developer not having to create workarounds for custom symbols that aren't unicode.

[Darkade]

I like to expand it to what the specification indicates; my only issue would be what it mentions about glyphs :

the number of variants should be identical for all glyphs.

What do you think about that @Darkade?

Shouldn't that be a concern for the typeface dev, tho?

[Hixie]

Thanks for fixing my mistake. :-)

[Darkade]

Wouldn't have noticed except for really wanting to use a very specific font 😹