improve flexibility of limactl shell cmd

[olamilekan000]

change improves the flexibility of the limactl shell command when used with the 
--preserve-env flag by validating environment variable patterns to only 
allow alphanumeric characters and underscores also allowing 
more flexible environment variable filtering.

Fixes
#4036

[AkihiroSuda]

The PR description is not clear to explain what is improved

[jandubois]

I haven't actually run the code, but from reading the changes, it doesn't look like it implements the modified allow list semantics.

It is also missing tests for the changes.

[jandubois]

Please run your tests before pushing to the PR:

--- FAIL: TestFilterEnvironment (0.00s)
    --- FAIL: TestFilterEnvironment/allowlist (0.00s)
        envutil_test.go:173: assertion failed: 9 (int) != 2 (int)
FAIL
FAIL	github.com/lima-vm/lima/v2/pkg/envutil	0.188s

[jandubois]

I think the tests need another round of simplification.