Extend ToBoolean function so it casts "t" to true and "f" to false.

[DavIvek]

User from community requested this extension for ToBoolean function. Since, e.g., postgresql COPY to CSV will record true values as "t" and false values as "f".
We also might need to update documentation so it's clear what values ToBoolean casts to true or false.

[master < Task] PR

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Release note: Strings "t" and "f" are now resolved into true and false.

closes #1538

[DavIvek]

NOTE: ToBoolean(t) will still return an error since it's resolved as a symbol. It doesn't make sense to me to resolve it as a boolean value.
The same thing is with boolean values not being resolved as symbols.
For example let's say we want to match a node from a database:
MATCH (truE:Node) return truE;
After executing above query we will get n rows (n being a number of nodes with label Node) of true values. truE won't get resolved into a symbol.

[imilinovic]

Looks good