Add FiraCode progress bars glyphs support

[asciimoth]

This adds support for progress bar glyph sets that need distinct empty/filled border characters, such as the Unicode progress bar glyphs provided by Fira Code.

Changes:

• Extends the bar template element with optional 6th and 7th arguments:
  • 6th: empty-state left border
  • 7th: finished-state right border
• Keeps existing 5-argument bar templates fully backward compatible.
• Adds an opt-in UNICODE_PROGRESS_BAR=true gate (as suggested by FiraCode readme) for using the Unicode progress bar glyphs as the implicit default template.
• Documents the new template arguments and environment variable.

[cheggaaa]

Thanks for the PR!

One thing I'd change: the env var currently swaps the whole default template, so it does nothing for Full, Simple, or a user's own template. Simpler and more complete — let it swap defaultBarEls instead:

var firaBarEls = [barElementCount]string{...}

func init() {
	if os.Getenv("UNICODE_PROGRESS_BAR") == "true" { / maybe also to check for "1", etc?
		defaultBarEls = firaBarEls
	}
}

[asciimoth]

I choose to preserve only "UNICODE_PROGRESS_BAR=true" and not "1" or "yes" because it may lead to situation when some apps understand specific env value and other (more strict) ignore it.

[cheggaaa]

Had a small review with an llm and found one bug:

args 6/7 now fall back to defaultBarEls[i], so a fully-specified 5-arg bar still inherits the Fira extras. With UNICODE_PROGRESS_BAR=true:

{{bar . "<" "=" ">" "." ">"}}   →   "........>"

The user's < left border is replaced by U+EE00 whenever the bar is empty — so the env var breaks existing custom bars, including the colored form we document. TestUnicodeProgressBarEnvDoesNotOverrideExplicitBarArgs misses it because it passes all seven args with "" "", which is the one case that isn't at risk.

Fix: take the extras only when no bar args were passed at all.

case i < barLeftEmpty:
	arg = argsH.getNotEmptyOr(i, defaultBarEls[i])
case len(args) == 0:
	arg = defaultBarEls[i] // extras come with the default glyph set only
default:
	arg = argsH.getOr(i, "")

[cheggaaa]

Thank you! v3.2.0 is out