feat(viewer): add Text, Markdown and Source Code viewer (read-only)

[ShGKme]

• Based on feat(viewer): add PDF viewer #784
• Only the last commit is related

☑️ Resolves

• A part of: Open PDF & text/markdown in-app #256

Includes features:

• File types:
  • .md with extended markdown support
  • .txt with auto link only
  • Different source code files with monospaced font (depends on the server's mimetype detection)
• Switch between 900px / fullwidth modes
• Copying text content
• Navigating/selecting like in a readonly contenteditable area
• Handling empty files

🖼️ Screenshots

Text file

🌕 Light	🌑 Dark

Markdown file

🌕 Light	🌑 Dark

Full-width mode

Soft-wrap - MD

Enabled	Disabled

Code (no highlighting)

🌕 Light	🌑 Dark

Empty file

🌕 Light	🌑 Dark

Copy content

Navigation

The text is within contenteditable, so a user can select/copy/navigate like in a text editor. But it is readonly.

[ShGKme]

Added many fixes separated into small commits. See PR description for new images.

• More types for code from the suggestion
• Changed icon from the suggestion
• Fix Copy button (it wasn't actually implemented, lol)
• Updated design to handle light theme
• Improved wrap/non-wrap text view + added a control for it
• Made menubar sticky on top instead of fixed on the corner
• Some minor UI changes

[Antreesy]

Don't know where it belongs code-wise, but I faced the caching issue:

• File is changed on server;
• Client is still keeping the old version (was empty for me);

Maybe could be done in a follow-up, but how can it better be done? Sync button, null cache time?

[ShGKme]

Hmm, a text file has no ETag change after its content was changed... So technically the cached value is valid...

[ShGKme]

Checking with the office team

[ShGKme]

Works fine if reopen after some short time, just not immediately after is was changes in the web-editor.

[ShGKme]

• Fixed regression with no code format
• Moved button to the right (removed 900px limitation)

[Antreesy]

Good from my POV