feat: Add column helper methods

[Techassi]

This PR adds the following methods:

• add_column
• add_column_if
• add_columns
• add_columns_if

Their use is documented via doc comments.

[codecov-commenter]

Codecov Report

Merging #126 (deed3e1) into main (af3924c) will decrease coverage by 1.98%.
The diff coverage is 34.48%.

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@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   96.36%   94.39%   -1.98%     
==========================================
  Files          18       18              
  Lines        1763     1819      +56     
==========================================
+ Hits         1699     1717      +18     
- Misses         64      102      +38     

Files Changed	Coverage Δ
src/table.rs	78.29% <34.48%> (-7.31%)	⬇️

[Techassi]

Any update on this @Nukesor?

[Nukesor]

Hey.

I actually don't feel comfortable to merge this. Column handling will stay implicit under the hood, but this MR gives the illusion that columns can (and should be) handled explicitely.

Instead of working on a workaround for the flaws of the current architectural design, I would rather like to do a full redesign on comfy-table's column handling.

This will probably require a lot of work, quite a bit of testing and several breaking changes in the API, but in the end it'll be worth it to have it properly planned and designed.

I have to honestly say that I'm currently lacking the motivation to do such a big change on this project, since I'm quite happy with the way it works right now (even though I see and underestand the flaws). I'll eventually get to it, but right now I'm caught up with work, some private matters (e.g. my SO and I are planning to move together) and a few other projects that captured (or require) my attention^^.

[Nukesor]

Closed due to #144