feat: add config option editor.statusline.unobtrusive to make statusline not block the last line of text

[nikitarevenco]

per popular request: How to disable / remove statusline? #7934

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This PR adds a new editor.statusline.unobtrusive (boolean) option

Previously, the statusline would always block one line of the screen. But now, if this option is set then the statusline elements will be overlaid on top of the bottom line of text without blocking that line

Showcase

With the following config, the text on line 23 is not visible.

[editor.statusline]
unobtrusive = false # default
center = []
left = []
right = ["spinner", "file-name", "diagnostics"]

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And setting unobtrusive = true makes the text on line 23 visible.

[editor.statusline]
unobtrusive = true
center = []
left = []
right = ["spinner", "file-name", "diagnostics"]

Notes

• The background color of the status line gets applied regardless, this is required because otherwise with multiple vertically stacked buffers it is very hard to tell where one buffer ends and another one starts:

Since people use their text editors with other software like multiplexers, having an entire line blocking the text isn't good for integrations.

With this config, you can hide all of the status line's elements:

[editor.statusline]
unobtrusive = true
center = []
left = []
right = []

Most notable changes are in these files:

• In the render method in helix-term/src/ui/statusline.rs:52
• In the inner_area and inner_height methods in helix-view/src/view.rs:193

Most other changes were just passing the unobtrusive_statusline argument

• This is my first time writing any Rust code, just saying in case I broke some best practices or something :)