ungron: accept tabs as whitespace alongside spaces

[ChrisJr404]

Closes #129.

The reporter wants ungron to handle input where statements are aligned with tabs, e.g. output from a tool that pads keys to a common column:

foo.bar    = "val1";
foo.hijklm = "val2";
foo.quxz   = "val3";

On master, ungronning that input fails — the tab between the path and = makes the lexer fall through lexStatement's switch into the typError default, because the dispatch only matches ' ' and '='.

This:

• Adds '\t' to the lexStatement dispatch case so a path followed by a tab routes to lexValue (the same way a space-separated path does).
• Changes the three l.acceptRun(" ") whitespace-skip calls in lexValue to l.acceptRun(" \t") so tabs around = and before ; are also consumed.
• Updates the grammar comment from Path Space* "=" Space* to Path WS* "=" WS* and defines WS ::= ' ' | '\t'.
• Adds two TestLex table cases: a tab-only "json\t=\t1;" and a mixed-whitespace "json.foo \t = \t \"bar\"\t;".

Verified ./script/test and a manual round-trip on the reporter's example:

$ printf 'foo.bar\t= "val1";\nfoo.hijklm\t= "val2";\nfoo.quxz\t= "val3";\n' | ./gron -u
{
  "foo": {
    "bar": "val1",
    "hijklm": "val2",
    "quxz": "val3"
  }
}