FILE support

[kyleoneill]

Need to support the ability to open a file in a test. Files should be handled depending on their format, examples include

• .txt - Read the file contents into a string
• .json - Read the file contents into a json object
• default - Read the file contents into a Vec<u8>

Will need to make sure that relative file pathing works. Ex, if a test is run like chimerascript run -f ./some/path/my_test.chs and a test contains a file path like foo = FILE ../files/foo.txt then the file path needs to be resolved as ./some/files/foo.txt