Copy files from Go 1.19.2

[pstibrany]

This PR copies latest version of regexp package from Go 1.19.2 into main.

[bboreham]

If I try this I get:

% go test ./...
# github.com/grafana/regexp
package github.com/grafana/regexp (test)
        exec_test.go:11:2: use of internal package internal/testenv not allowed
FAIL    github.com/grafana/regexp [setup failed]
ok      github.com/grafana/regexp/syntax        0.524s
FAIL

[pstibrany]

If I try this I get:

% go test ./...
# github.com/grafana/regexp
package github.com/grafana/regexp (test)
        exec_test.go:11:2: use of internal package internal/testenv not allowed
FAIL    github.com/grafana/regexp [setup failed]
ok      github.com/grafana/regexp/syntax        0.524s
FAIL

That's my bad, fixing in #6 (for speedup-golang-1.19.2 branch only, you fixed it on main)

I haven't realized that you have removed usage of internal on main when copying latest version of regexp package from Go 1.19.2. I mistakenly thought main branch is without changes.