lang: Add fileexists function

[radeksimko]

I treat this is as "ready for review" functionally.

Addresses #10878, but will not close it until the v0.12-dev branch is merged to master.

[apparentlymart]

The implementation looks good to me!

Historically though we've always named functions with words all strung together with no delimiters, like fileexists. While that precedent is certainly not ideal for readability, I think it's better that we stick with it so we don't force users to remember which functions use underscores and which ones don't.

I left some inline feedback about the docs.

[apparentlymart]

Given the example above of using this to conditionally call file(), perhaps it'd be nice to include the opposite link here too, so readers of the docs for the file function can find this function and see that conditional reading pattern.

[apparentlymart]

Let's use a period at the end of this sentence for consistency with how the others are written.

[apparentlymart]

I think it's important to also mention here two restrictions:

Functions are evaluated during configuration parsing rather than at apply time, so this function can only be used with files that are already present on disk before Terraform takes any actions.

This function works only with regular files. If used with a directory, FIFO, or other special mode, it will return an error.

[apparentlymart]

My previous response still stands, but v0.12-dev is no longer active since it's been merged into master, so unfortunately these changes will need to be cherry-picked onto a new branch from the latest master 😞

[apparentlymart]

Looks good, thanks!

(And sorry for the collision with the merge down to master.)

[ghost]

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