Added module overview doc comment to lexer

lexer/doc.go

@@ -0,0 +1,6 @@
+// Package lexer implements the lexical analysis stage of the FSPL compiler.
+// Its job is to convert text into a series of tokens (lexemes) which are then
+// passed to other parts of the compiler to be interpreted into more complex
+// data structures. The lexer is able to read in new tokens as needed instead of
+// reading them in all at once.
+package lexer