[kast-ast] [feature] Add ability to parse tokens without parsing new syntax-defs

[bhavyakukkar]

There is no way to prevent the parser from parsing syntax-definitions.

At current, the only public functions that can be used to receive an AST from a source-file are read_syntax to read syntax definitions and parse to parse a source-file into syntax-trees (including new syntax-definitions) from a set of existing syntax-definitions.
There is no way to just use an existing set of syntax-definitions to parse a source-file, without the ability to parse new syntax-definitions.

From the perspective of wanting to use kast-ast to design parsers for languages other than kast, it would make sense to have a switch or a different function to disable the ability to read syntax-definitions from the source file. This would make it easier to decouple the parser into a parser generator and the parser that it generates, i.e., the syntax-definition