Add string literal completion

This also adds the behavior that \ and " signs inside of string literals
are automatically escaped since they would cause compiler errors
otherwise.

Fix #1001398
Add character literal completion

As for string literals, \ signs are automatically escaped to prevent
compiler errors.

Furthermore it turned out that the current partitioning strategy doesn't
work perfectly together with this component. Two consecutive apostrophes
aren't handled as their own partition since they are not valid Scala
syntax. Thus, the behavior that should only applied to existing
character literals is placed in the same component and must be moved if
the partitioning strategy for character literals ever changes.

Fix #1001403
Add option value for auto escaping strings
Add option value for auto escaping character literals