Change license to LGPLv2.1+

interp: Don't wrap Bool/Text values with value()

interp: Catch interrupts in System Read

interp: Implement conditionals and add example to code.txt

interp: Add Equals verb to Text

Add True/False as booleans

interp: Loop program until System Exit

interp: Check arguments for Text Append and System Print

interp: Raise an error on invalid value references

interp: Raise an error if a verb doesn't exist

interp: Use exceptions for interpreter errors

interp: Replace 'if' with 'conditional' in error

interp: Return the last command's result

interp: Make environment an Interpreter variable

interp: Put interpreter in its own class

interp: Replace system module with a class

interp: Prefix verbs in classes with verb_

interp: Use class for text values

interp: Rename run_if to run_conditional

ast_types: Move AST data structures to ast_types

interp: Add run_ast function

interp: Move interpreter here from main