Simplification of grammar that encodes mutations with the same productions as reactions

[Wonseo-C]

This PR:

• Updates the Lingua Franca grammar to delete the Mutation class, and merge it with the Reaction class. (They have the similar properties Trigger, Source, Effect, Code)
• Updates the TypeScript code generator to uses a WritablePort for mutations

[hokeun]

@lhstrh, I talked to @Wonseo-C and we found that Reaction and Mutation have a lot in common. Do you know if we can define a parent of these two objects in LinguaFranca.xtext? If we have a sort of "parent class" of Reaction and Mutation, we will be able to write code a lot cleaner without repeating the same code. :) For example, we can define common functions used by procedures generating codes for reactions and mutations.

[lhstrh]

Looks good! Thanks for these fixes!

[hokeun]

Great work @Wonseo-C ! Thanks for the review @lhstrh !