Add plugin for Please (#6568)

plugins/please/README.md

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+# please plugin
+
+[Please](https://please.build) is a cross-language build system with an emphasis on
+high performance, extensibility and reproduceability. It supports a number of popular
+languages and can automate nearly any aspect of your build process.
+
+This plugin adds autocomplete and major aliases for `plz`, the command line tool for
+Please.
+
+To use it, add `please` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... please)
+```
+
+## Aliases
+
+| Alias | Command     |
+|-------|-------------|
+| `pb`  | `plz build` |
+| `pt`  | `plz test`  |
+| `pw`  | `plz watch` |
+
+## Maintainer
+
+[@thought-machine](https://github.com/thought-machine)

plugins/please/please.plugin.zsh

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+if (( $+commands[plz] )); then
+    source <(plz --completion_script)
+fi
+
+alias pb='plz build'
+alias pt='plz test'
+alias pw='plz watch'