refactor parser entry points and extract a few methods out

This commit contains three logical changes:

1. Split `templateStatSeq` into two methods as we need more reliable
   parsing of template body alone for new `parseStats` entry point.

2. Add new parser entry point called `parseStats` which is aimed towards
   use in tools that require parsing of Scala code that can be written
   inside of a template.

   Such functionality is required for parsing lines in repl, parsing
   code through toolbox, parsing and running scala scripts and lastly
   for quasiquotes. All of them are refactored to use this very method
   in the next commits.

   A new method called `templateStatsCompat` is also added to make this
   commit pass the tests but it's a temporary hack that will be removed
   in next commit in favor of `parseStats`.

3. Extract out a few methods like `isCaseDefStart`, `expectedMsgTemplate`
   and `parseRule`. These are needed to override parser behaviour in
   updated quasiquotes parser (see next commits).

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