Current Terminal size added in About popup

[Swarnim114]

closes #1536

What does this PR do, and why?

Adds a Terminal size label under the "Detected Environment " in About popup

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes Add current terminal window size to About popup #1536
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[Swarnim114]

@zulipbot label "PR needs review"

[Swarnim114]

Hi @neiljp,

Sorry for the mess-up in that PR earlier. I'm new to open source development. Thanks for your understanding, and please continue guiding me.