lex: add support for implementation blocks of conditions

[jafl]

In some lexers, all the rules for a condition are grouped together as: { ... }

It is useful to be able to jump to these blocks.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c26e151) 85.38% compared to head (252e692) 85.38%.

Files	Patch %	Lines
optlib/lex.c	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3937 +/- ## ========================================== - Coverage 85.38% 85.38% -0.01% ========================================== Files 234 234 Lines 56200 56201 +1 ========================================== Hits 47987 47987 - Misses 8213 8214 +1 ```

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[masatake]

I wonder whether we should capture it as a definition or reference tag.

Tagging B_ENVIRONMENT as grouping role of condition kind is my alternative idea.

[masatake]

I wonder whether we should capture it as a definition or reference tag.

[jafl]

I wonder whether we should capture it as a definition or reference tag.

How would that change the code and the output?

[masatake]

How would that change the code and the output?

I'll make a pull request based on your pull request to demonstrate tagging ' {..}` with a reference tag. Please, give me time. I have trouble with my health.

[jafl]

Closing in favor of #3939

[jafl]

And I hope you feel better soon!