optlib: allow users to set fields and check extras defined in a foreign language

[masatake]

In addition, this pull request allows you to fill out a field for a foreign tag from optscript.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.30769% with 8 lines in your changes missing coverage. Please review.

Project coverage is 85.85%. Comparing base (23c6e36) to head (d87078d). Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
main/field.c	90.69%	4 Missing :warning:
optlib/scss.c	0.00%	3 Missing :warning:
main/options.c	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3960      +/-   ##
==========================================
+ Coverage   85.84%   85.85%   +0.01%     
==========================================
  Files         244      244              
  Lines       62904    62986      +82     
==========================================
+ Hits        53997    54079      +82     
  Misses       8907     8907              

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[masatake]

The ways to support boolean fields are not consistent in various aspects.

[masatake]

f3246a26e5cf2ffbce74723a58793192ccaa24d3 and 1f073fc99fbc999b4b1bb9524bd82dd945c3ad6f can be cherry-pick'ed.

[masatake]

We can merge c428bd3 ... 9b3f25f.

d15cc30 needs a test case.

[masatake]

I added test cases. The top half of this pull request can be merged. I will make a pull request.

[masatake]

This pull request was for a Texinfo parser. However, this pull request may be useful for SCSS parser, too.