Co-authored-by: NPolina Sokolova <polinasok@users.noreply.github.com>

This change update liner to the latest release that adds support to
delete the next and the previous word with Alt-d and Alt-BackSpace
plus a small amount of fixes.

Formally define these types and document their meaning.
We will auto-generate the dlv-dap documentation from these Go type doc.

mapToStruct is a helper that sets the given struct's fields with the
info in map[string]interface{} (launch/attach's Arguments). We achieve
this by reencoding map[string]interface{} to json and decoding back to
the target struct. If go-dap left the implementation-specific arguments
as json.RawMessage and let the implementation decode as needed, this
reencoding could've been avoided.

encoding/json itself does not have mean to enforce required fields.
There was a test case that checks substitutePath elements must set
both from/to fields. Path.UnmarshalJSON implements the check.
I am not yet sure about the need for distinction between missing
'from/to' and empty strings yet. (empty value is useful when dealing with
a binary built with trimpath, right?)

A minor behavior change - previously, if noDebug is not a boolean type,
we ignored the attribute silently. Since we use json decoding, any
mismatched types will cause an error and this non-boolean type noDebug
attribute will result in launch failure.

This patch implements fuzzy searching for tab completions in the
terminal client. Under the hood it is using a trie data structure
(https://en.wikipedia.org/wiki/Trie) to perform very fast prefix / fuzzy
searches.

This removes indirect dependencies from go.mod, and
includes the fix for the missing -help flag info.

The latest cobra release is v1.2.1. Given that there were
minor security-related dependency cleanup during v1.2 release,
I was tempted to pick up the latest version, but that caused
dependency updates in golang.org/x/sys and golang.org/x/tools
which may be too recent (golang.org/x/* follow the go's release
support policy, so recent versions may not be compatible with
go versions beyond go's official version support policy).

Verified that dlv still builds with go1.12.x.
(go1.12 is the oldest version of go that can build the latest delve already).

$ go get -d github.com/spf13/cobra@v1.1.3
$ go mod tidy
$ go mod vendor
$ go run _scripts/gen-usage-docs.go

* go.mod: update google/go-dap to 0.5.0

* Go mod tidy
Co-authored-by: NPolina Sokolova <polinasok@users.noreply.github.com>

Co-authored-by: NPolina Sokolova <polinasok@users.noreply.github.com>

Pulls in v0.3.0 of google/go-dap; this version has some fixes and API
improvements which service/dap will be able to leverage to clean up some
code.

* vendor: update starlark

* terminal: use new mechanism to cancel starlark threads

See: https://github.com/google/starlark-go/pull/298

It is only used in benchmarks and 'go test' now has command line
options to enable profiling as appropriate (but maybe it didn't in
2014).

Fixes #2037

Fixes #2026

Remove gitignore rule preventing vendor/modules.txt file from being
commited, add check that the vendor directory is synchronized.