[lldb-dap] Add `debugAdapterEnv` for `attach` requests & improve regex (#157980)
# Changes
#153536 added a new debug configuration field called `debugAdapterEnv`
and enabled it in `launch.json` **for `launch` requests**. This patch
enables the same for **`attach` requests**.
This patch also improves the regex used in this field, i.e. shortens it
and fixes the double backslashes (`\\`) in `debug-adapter-factory.ts`
(note: the ones in `package.json` need the double backslashes).
# Test
Manually tested the following values in `attach` requests (so that we
are testing both changes at the same time):
```
// Accepted
"debugAdapterEnv": [
"AAA=BBB",
],
"debugAdapterEnv": [
"AAA=",
],
"debugAdapterEnv": [
"AAA",
],
// Rejected
"debugAdapterEnv": [
"=AAA",
],
"debugAdapterEnv": [
"=",
],
"debugAdapterEnv": [
"",
],
```