Add fold regions to Tables and TableArrays

[tpikonen]

Tables and TableArrays are folded when foldmethod=syntax is set.

The folds are implemented by a single regex which matches the table and table array headers. This has to replicate a large part of the syntax for TOML keys, so the regex becomes a bit involved. Fold levels are limited to 1 for a table and 0 for the part before the first table. This is because dictionary depth cannot be parsed from the header with a regex and cannot be communicated to vim by syntax statements anyway.

There is an edge case where a valid inline array looks like a table header, and is detected as such. This is hopefully rare enough in practice to not matter. A demonstration of this is added to test.toml.

This perhaps could have been implemented with a clever use of contains, containedin etc., but that would require a larger refactoring and maybe fixing of other bugs such as #58.

Fixes #63.

[A4-Tacks]

Working on large files using syntaxfold may result in performance issues
Or it may be due to syn-sync causing abnormal folding

Considering foldexpr?
:set foldmethod=expr
:set foldexpr=getline(nextnonblank(v:lnum+1))=~'^['?'<1':1