fix(tag parser): use %% to get a literal percent sign (%)

[xi]

As described in the lemonbar docs: https://github.com/LemonBoy/bar#formatting

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• Refactor
• Feature
• Bug Fix
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Description

The lemonbar documentation says that you can

Use %% to get a literal percent sign (%).

That was not implemented in polybar.

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[patrick96]

This is going to break a lot of things. Take for example this config:

[bar/example]
modules-left = text

[module/text]
type = custom/text
format = Text%

For this, polybar will produce the string %{l}Text%%{PR} which is then rendered as Text%{PR}, even though we would expect it to render as Text%.

The literal percentage sign in format unintentionally escapes formatting tags that follow the text module.

I'm not sure adding literal percentage signs like this is possible at all in a backwards-compatible way.