Rename command and brand name

[magodo]

Per #357, this PR renames the brand name of the tool from Azure Terrafy to Microsoft Azure Export for Terraform, also rename the command name from aztfy to aztfexport..

So we get:

aztfy on  rename via 🐹 v1.20.1
💤  grep --exclude-dir=.git/ --exclude-dir=.idea -ri aztfy
azure-pipelines.yml:              gitHubConnection: "github_aztfy"

aztfy on  rename via 🐹 v1.20.1
💤  grep --exclude-dir=.git/ --exclude-dir=.idea -ri terrafy

aztfy on  rename via 🐹 v1.20.1
💢  grep --exclude-dir=.git/ --exclude-dir=.idea -ri terrafies

Other than the command gets renamed, other breaking changes introduced by this PR include:

• Resource mapping file name: aztfyResourceMapping.json -> aztfexportResourceMapping.json
• Skipped resource list file name: aztfySkippedResources.txt -> aztfexportSkippedResources.txt
• Config directory name: .aztfy -> .aztfexport

Some TODO items after this PR:

• Rename the repo
• winget pacakge name changed from Azure.Aztfy to Azure.AztfExport, which requires us manually send a PR towards winget to put things at the new location
• homebrew also requires a manual PR to reflect the new formular name

[stemaMSFT]

Thank you!