term: improve performance by caching redundant can_show_color calls

[ttytm]

When using the term module for CLI projects, functions like colorize and other functions that check first for the ability to display colors can be called quite a lot. Currently the value of can_show_color_on_stdout()->supports_escape_sequences() is re-processed on every call of these functions.

v/vlib/term/term.v

Lines 184 to 202 in 5bba92a

	@[manualfree]
	fn supports_escape_sequences(fd int) bool {
	vcolors_override := os.getenv('VCOLORS')
	defer {
	unsafe { vcolors_override.free() }
	}
	if vcolors_override == 'always' {
	return true
	}
	if vcolors_override == 'never' {
	return false
	}
	env_term := os.getenv('TERM')
	defer {
	unsafe { env_term.free() }
	}
	if env_term == 'dumb' {
	return false
	}

The changes add a cache for the can_show_color status which results in a speedup of 100%+.

import term
import time

sw := time.new_stopwatch()

for _ in 0 .. 10_000 {
	term.ok_message('my message')
	term.colorize(term.red, 'red')
}

println(sw.elapsed())

_{Huly®: V_0.6-21336}

[spytheman]

Excellent work.