Harmonize strings delimiters inside the code, to free " or ' as compression token

[Siorki]

Currently, characters 34 " and 39 ' are banned as tokens for crusher / packer stages.
The rationale behind that is that if one is present in the input, the other one will be needed as the delimiter for the packed string.

However, if neither of them is present in the string, one will still be used as delimiter, while the other one could become a token. This adds a bit of complexity upon building the token list as the algorithm will have to determine which one will produce the longest ranges in the character class.
Having one or the other will not impact the crusher though.

[Siorki]

Complement : support for backquotes (ES6 template string) if ES6 flag (#54) is on.

[Siorki]

Extended the issue scope, hence the new title.

Following initial work on this issue, the preprocessor now recognizes strings in the input code. This might be reused for other tasks (#13, #57).
With ES6 we now have 3 delimiters : ' " `. Changing them in strings inside the input is an opportunity to free one either as a compression token, or to wrap the packed string without having to escape other occurrences inside. As this is performed before the crusher, there is no easy way to know whether one extra token will be useful (in 1k format, it usually isn't).

Suggested algorithm :

• identify all strings inside the code, along with their current delimiter and `"' present inside
• for each delimiter that can wrap the packed code, elaborate a minimal input
• choose the delimiter that produces the shortest input (least escapes needed)
• choose the delimiter that produces the shortest input (least escapes needed) while freeing one token (if possible)
• create a branch for each so the need for the extra token will be determined after the crusher does its work, by comparing packed lengths

[Siorki]

Current implementation only addresses the first three bullet points above.
Improvements have been deferred to separate issues :

• tiebreaker between two possible quotes : String delimiter : optimize tiebreaker #60
• refactoring strings to free one token (bullet points 4 and 5 above) : String delimiter : free one quote to use as token #61