[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": [
                "",
            ],
```

0 files changed, 0 insertions, 0 deletions