Provide filter-buffer-substring-function in gfm-view-mode

[muffinmad]

Description

gfm-view-mode is used by eglot to show symbol information in markdown format. E.g. the LSP server send something like this as the documentation:

```python
foo(bar=None):
```

```
Docstring for foo
```

eglot-help-at-point will show this text nicely fontified by gfm-view-mode.

The first line (which usually is the signature info) of that text is also used as the eldoc message to show in the echo area. The problem is that the first line is actually invisible codeblock marker.

This PR provides the function that return buffer substring without codeblock markers and can be used as value of the filter-buffer-substring-function variable. This way fontified signature info can be shown in the echo area.

Similar behavior is used by the diff-mode and the backtrace-mode.

Thanks!

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

[syohex]

@muffinmad Thanks for PR. However I suppose this is not enough

• filter-buffer-substring-function should be set both markdown-view-mode and gfm-view-mode
• It should filter not only code block(triple backticks) but also other invisible characters

I have sent PR #494. How about it ?

[muffinmad]

I have sent PR #494. How about it ?

Perfect! Thanks!

[muffinmad]

Closing in favour of #494

[syohex]

Thanks for checking. I have merged #494. Please check latest version.

[muffinmad]

Thanks for checking. I have merged #494. Please check latest version.

Works as expected. Great work! Thanks!