Introduces literals and type checker

[momeemt]

The compiler now treats the type of all tokens as Text. However, there are more kinds of data in general documents.
For instance Path, Integer, Record and Enum.
I felt so difficult and too complex to implement large amount of validations and castings when developing Brack plugins.
We'll introduce the brack in literals and handy type checker to make it easy to build plugins.

Literal

The literal can be written as `DATA.
For example, `123 is inferred as an Integer type, and `{ foo: 123 } is inferred as Record (to be precise, its type is Record{ foo: 123 }). As an exception, the statement whose first character is a dot (.) is inferred as Path (like Nix).