Added heading-style for all the four themes

[mounilKshah]

What does this PR do?
This PR emphasises the headings present in the message.
Fixes #1095

Discussion on the dev community:
#zulip-terminal> Emphasising heading styles #T1095

Tested?

• Manually
• Existing tests (adapted, if necessary)
• New tests added (for any new behavior)
• Passed linting & tests (each commit)

Commit flow

• Only one commit which contains all the changes made and the test cases updated and added for heading-styles.

Notes & Questions

• Tests have been added for the heading styles.
• This PR resolves the #T1095
• At present, h1-h6 have the same implementation for a particular theme.

Visual changes

After implementing the heading-styles:

zt_dark

zt_light

zt_blue

gruvbox

[prah23]

Thanks for submitting this pull request, @mounilKshah.
I'd suggest modifying the commits (and their messages) to suit this project's format. The tests for the changes are expected to be present in the same commit as the changes themselves. The current commit structure can be modified to lesser commits in a more coherent form. You can take a look at Zulip's commit discipline documentation for guidance with structuring your commits and our project's commit style section for suggestions on improving your commit messages.

[prah23]

Thanks for the update @mounilKshah, looks good to me. 👍

[neiljp]

@mounilKshah Thanks for working on this 👍 I've slightly simplified the commit text but have otherwise merged as you had it 🎉