x/tools/gopls: add .if and .not snippets for bool types

[hhhapz]

Is your feature request related to a problem? Please describe. The
introduction of postfix snippets has brought very convenient functionality to
gopls, especially when working outside of Goland. I wanted to contribute more
ideas for postfix completions inspired from what's already available from their
editors. In particular, I wanted to propose having a if! and not! prompts,
which do this (respectively):

(complex bool expression).if|
if (complex bool expression) {
	|
}

(expr).not|
!(expr)

Describe the solution you'd like
Implementing these in completion/postfix_snippets.go.

Describe alternatives you've considered
As a vim developper personally, I use
snippetsEmu. Other
potential alternatives are available in VSCode and Goland.

[findleyr]

CC @muirdm.

[findleyr]

Thanks for the suggestion. Muir might have some thoughts on this.

[hhhapz]

Wanted to add, that we could totally over-engineer the not by handling stuff like ==, >= ^, etc specially but I personally think it's overkill. Most of the time (at least personally) that this is useful is just negating a boolean var or a complex boolean expression surrounded by parenthesis.

In my mind, for the edge cases:

if x == y.not

should expand to

if x == !y

This helps keep the completion snippets more light weight, and if the user so desires, they can get the functionality by surrounding the expression with parentheses, or alternatively manually changing it.