feat: add column fitting commands

[dl-alexandre]

Summary

• add per-column width support so rendering, editing, scrolling, and mouse selection can respect fitted columns
• add :fit to size the selected column to its displayed contents and :fit width to spread visible columns across the terminal width
• cover the new fitting behavior with command and mouse-hit tests, and document the commands in the README

Testing

• go test ./...

Closes #20
Closes #21

[maaslalani]

Thank you this is great!

Would you be open to changing this to allow the user to manually set the column width?

I was thinking a :resize 20 command to change the current column to 20 width, etc... and then maybe having a :resize auto to fit content or :resize content, etc...

[dl-alexandre]

Updated the PR to use a command family.

Included:

• to set the current column width explicitly
• / to fit the current column to content
• to size the visible columns to the terminal width

I also updated the tests and README, and ? github.com/maaslalani/sheets [no test files]
ok github.com/maaslalani/sheets/internal/sheets (cached) is still passing.

[dl-alexandre]

Updated the PR to use a :resize command family.

Included:

• :resize 20 to set the current column width explicitly
• :resize auto / :resize content to fit the current column to content
• :resize width to size the visible columns to the terminal width

I also updated the tests and README, and go test ./... is still passing.

[erhhung]

Would be nice to have shortcut keys for manually resizing the current column, e.g.

Ctrl-, — 1-char narrower
Ctrl-. — 1-char wider
Ctrl-/ — :resize auto