refactor(text): replace Spans with Line

[joshka]

Line is a significantly better name over Spans as the plural causes
confusion and the type really is a representation of a line of text made
up of spans.

This is a backwards compatible version of the approach from
#175

There is a significant amount of code that uses the Spans type and
methods, so instead of just renaming it, we add a new type and replace
parameters that accepts a Spans with a parameter that accepts
Into<Line>.

Note that the examples have been intentionally left using Spans in
this commit to demonstrate the compiler warnings that will be emitted in
existing code.

Implementation notes:

• moves the Spans code to text::spans and publicly reexports on the text
  module. This makes the test in that module only relevant to the Spans
  type.
• adds a line module with a copy of the code and tests from Spans with a
  single addition: impl<'a> From<Spans<'a>> for Line<'a>
• adds tests for Spans (created and checked before refactoring)
• adds the same tests for Line
• updates all widget methods that accept and store Spans to instead
  store Line and accept Into<Line>

---

There is a second related commit on this PR to move text::Masked -> text::masked::Masked, with no changes.

---

Tagging @Eyesonjune18 and @TimerErTim for their input
This will likely require changes to be made on the changes in #149 (hopefully, just in Text and Line?)

I also updated the examples in a third commit. I wanted to make sure that backwards compatibility was maintained in a way that made sense first though so I intentionally kept the two commits separate. E.g.:

 1  warning: use of deprecated struct `ratatui::text::Spans`: Use `ratatui::text::Line` instead
   --> examples/panic.rs:29:20
    |
 29 | use ratatui::text::Spans;
    |                    ^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

I think the only breaking change on this is any code that specifically calls Text::into_iter() and then stores that in a variable or field of type IntoIter<Spans<'a>> rather than just letting type inference do its thing. This seems unlikely enough that it's probably not even worth noting.

[lthoerner]

Thanks for the PR rework, LGTM.

[sophacles]

I like where this ended up! LGTM

[sayanarijit]

Lookg good 👍

[TimerErTim]

LGTM
Great work and great idea to begin with!

[orhun]

LGTM 🦀