feat: add signed type and const array support to SDL parser

[edrthomas]

• Add signed keyword support to ElementaryType definition
• Add const qualifier support to ArrayDefinition rule (was missing)
• Update AST mapping files for new token types

[vectronic]

@edrthomas as discussed:

• signed int is an alias for int and I think is unnecessary in SDL.
• const qualifier for arrays is not defined in edition 1 of SDL. However in the current draft of edition 2 of SDL, const arrays are defined for computer array variables. Note that the use of const for parsed variables in general is (I hope) soon to be removed, so the concept of a const parsed array will not be required.

[edrthomas]

As discussed, let me close this PR for now.