Add import feature

[fly-lang]

Related to: Syntax: define package, imports and namespace

Implements the import logic feature

A single source file of Fly code must be able:

• to import one or more different packages,
• to access functions into the imported packages,
• to raise an error if two packages have the names.

Add import keywords to:

fly/include/Basic/TokenKinds.def

Line 113 in 35589d5

KEYWORD(auto)

[marcoromagnolo]

Import each package individually:
import "packageA"
import "packageB" as "B"
import "packageC" as "."

Import grouped packages:
import ("packageA", "packageB" as "B", "packageC" as ".")

[marcoromagnolo]

Resolved with pull request: #22